I have a model I've built that takes a polygon feature class converts it to points and the reverse geocodes to get a postal address. The issue is that the Reverse Geocode produces fields that act oddly with modelbuilder; I wanted to concatenate several fields to produce a single string address with all data. When I run the expression:
" ".join([!REV_Street!, !REV_City!, !REV_Region!, !REV_Postal!])
It says that the field is invalid, but when run outside of modelbuilder as a single Calculate Field geoprocess it works. How can I get the model to function?
I have tried in the model builder on my end (ArcGIS Pro 10.9.1), and it runs all good. Things I may have done a bit different from you if you did not mention or forgot to consider are:
" ".join((!REV_Street!, !REV_City!, !REV_Region!, !REV_Postal!))
It did not have any issue with me, so hope it also works for you.