Check permissions on the unit.
I had this recently with a USB/RS485 adapter.
When plugged in - it creates the port ttyUSB0 - this is in the /dev/folder
By default only members of the Dialout group have access to this to read and write.
As a test you can chmod it for the user you are running NR as, but this will be lost when you next unplug or restart
You need to add the NR running user to the Dialout group or add in a rules.d script to change the rights each time the device is initialized
This was my solution on Ubuntu on a laptop recently.
Even after all of this i could not get the node-red-contrib-modbus to workout - had to go to modbus-serial to get it to work.
Been running happily for 10 days in test mode polling an energy meter every 10 seconds.