When I have been performing a field calculator operation in ArcGIS pro using a table join and while calculating the field the error 999999 comes up and it says "Invalid column value [VALUE]. After inspecting my data it turned out that I had to increase the number of characters in my text field so the calculate field tool worked successfully.
I believe it would be beneficial to include something more descriptive as to why the tool failed as I sent a support ticket in and used up a lot of time for a relatively easy fix. something more like "too many characters in value [Value]" and maybe point out the specific feature or value where the field calculator failed.