This error message occurs when you try to add a nullable field to a non-geodatabase feature class. I'm wondering if your model is working on a temporary dataset in a folder scratch workspace when previously this was a geodatabase scratch workspace.
Something else I thought of is perhaps the field was there before (which makes the add field skip processing with a warning) and now it isn't there so it's now actually trying to create the field, causing the error mesage.
Since the model builder is 120+ processes to do this restructuring of the table, maybe this is a good opportunity to re-engineer your process using the Merge tool with a field map (created with Calculate Value) may be a more efficient and maintainable approach then adding, dropping, and calculating fields.
Thanks for the tips. Where I work the standard is still shapefiles and DBF tables so no part of the model touches anything geodatabase related.
In regards to the last suggestion, when I original made the model I tried to use the merge tool to field map and it didn't work. I found countless forum posts about how that was a broken tool in that sense and so I gave up on it. Does it now work correctly? I would LOVE to rebuild the model and cut it down to just a few steps! I'll have to spend an afternoon trying it!