As you can see from your debug image, the output payload is an object with a property named buffer.
Hover your mouse over the buffer property in the debug panel and some buttons will appear one of them being "copy path". Click the "copy path" button, then go to the buffer parser node and paste the path into the property field
Hey MaaAR! Like what Steve said, the MODBUS module has two outputs that output the same data in two different ways. The top node is meant to give a msg.payload that can be passed on to other nodes to be interacted with using the payload property. The bottom node is meant to feed out the raw data as a buffer for processing in case you need to do something like little Endian to float conversions. The way MODBUS processes data doesn't always produce a straight forward result in payload, which is why the second node exists.
Hence why you have to do as Steve points out and access the msg.buffer property with your parser instead of the msg.payload property, which as you can see in your screenshot doesn't exist on that node. Hopefully that helps and if so, give Steve's comment a mark as your answer so everyone knows you found it!