Try this using JSONata
[{"id":"874b0adc0efdef79","type":"inject","z":"bf9e1e33.030598","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"{\"payload\":{\"meta\":{\"code\":200},\"data\":{\"tracking\":{\"checkpoints\":[{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":null,\"country_name\":null,\"message\":\"Yanwen Pickup scan\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_002\",\"subtag_message\":\"Acceptance scan\",\"checkpoint_time\":\"2022-05-30T08:08:40+08:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"PU10\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":null,\"country_name\":null,\"message\":\"Processing information input\",\"country_iso3\":null,\"tag\":\"InfoReceived\",\"subtag\":\"InfoReceived_001\",\"subtag_message\":\"Info Received\",\"checkpoint_time\":\"2022-05-30T20:08:40+08:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"SC10\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":null,\"country_name\":null,\"message\":\"Last mile=> Norway Post, number LM002842896NO\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_001\",\"subtag_message\":\"In Transit\",\"checkpoint_time\":\"2022-06-01T05:26:41+08:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"SC40\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":null,\"country_name\":null,\"message\":\"Port of departure - Departure\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_007\",\"subtag_message\":\"Departure Scan\",\"checkpoint_time\":\"2022-06-04T12:01:00+08:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LH20\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":null,\"country_name\":null,\"message\":\"Port of destination - Arrival\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_003\",\"subtag_message\":\"Arrival scan\",\"checkpoint_time\":\"2022-06-05T13:38:00+08:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LH30\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":\"Norway\",\"country_name\":null,\"message\":\"Package is in transit.\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_004\",\"subtag_message\":\"Arrived at destination country\",\"checkpoint_time\":\"2022-06-10T01:43:52+01:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LM10\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":\"Norway\",\"country_name\":null,\"message\":\"Package is in transit.\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_001\",\"subtag_message\":\"In Transit\",\"checkpoint_time\":\"2022-06-10T01:48:32+01:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LM12\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":\"Norway\",\"country_name\":null,\"message\":\"Package is in transit.\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_001\",\"subtag_message\":\"In Transit\",\"checkpoint_time\":\"2022-06-10T07:06:36+01:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LM12\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":\"Norway\",\"country_name\":null,\"message\":\"Package is in transit.\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_001\",\"subtag_message\":\"In Transit\",\"checkpoint_time\":\"2022-06-13T06:36:01+01:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LM12\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":\"Norway\",\"country_name\":null,\"message\":\"Package is in transit.\",\"country_iso3\":null,\"tag\":\"InTransit\",\"subtag\":\"InTransit_001\",\"subtag_message\":\"In Transit\",\"checkpoint_time\":\"2022-06-13T09:20:29+01:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LM12\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":\"Norway\",\"country_name\":null,\"message\":\"Package has arrived at pickup point.\",\"country_iso3\":null,\"tag\":\"AvailableForPickup\",\"subtag\":\"AvailableForPickup_001\",\"subtag_message\":\"Available for pickup\",\"checkpoint_time\":\"2022-06-13T16:47:26+01:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LM30\"},{\"slug\":\"yanwen\",\"city\":null,\"created_at\":\"2022-06-25T07:57:05+00:00\",\"location\":null,\"country_name\":null,\"message\":\"Package has been delivered.\",\"country_iso3\":null,\"tag\":\"Delivered\",\"subtag\":\"Delivered_001\",\"subtag_message\":\"Delivered\",\"checkpoint_time\":\"2022-06-18T09:26:24+01:00\",\"coordinates\":[],\"state\":null,\"zip\":null,\"raw_tag\":\"LM40\"}]}}}}","payloadType":"json","x":90,"y":60,"wires":[["114c4fe7.8520a8"]]},{"id":"114c4fe7.8520a8","type":"change","z":"bf9e1e33.030598","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"$substringAfter($$.payload.payload.data.tracking.checkpoints[$contains(message,\"Last mile=>\")].message, \"number \")","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":260,"y":60,"wires":[["939cb148.16935"]]},{"id":"939cb148.16935","type":"debug","z":"bf9e1e33.030598","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":510,"y":60,"wires":[]}]
$substringAfter(
$$.payload.payload.data.tracking.checkpoints[$contains(message,"Last mile=>")].message,
"number "
)