Use the debug node and sidebar to get the path, using the copy path button next to voltage. It appears when you hover the mouse to right of property name.
it should be msg.payload.voltage. if you use the copy path method you will get payload.voltage, you would then append msg. to front if using it in a function node.
Feed the message into a debug node and screenshot so we can check what you have. You can paste an image here. Also show us how you are trying to access it that is getting undefined and how you know it is undefined.
Change the debug node to show msg.payload, see what is there, then check that the layout of the payload is correct for the influx node. The help text for the influx node tells you what is allowed. Note that nested objects are not allowed in influx. You must decide what fields and tags you want in the database and build the payload accordingly.
That is because you have not formatted the payload as required by the node. Have you read the help text and decided what fields and tags you want? If you don't know what influx fields and tags are then you need to read the influx docs.
As you have it, it will add a record each time new data arrives via MQTT. If you want to slow that down (or speed it up) then you could change the rate at which the publishing device sends data. Alternatively to slow it down you could configure a Delay node configured in Rate Limit mode, with Drop Intermediate Messages.