realize this is answered, but I wanted to add some context. 1Code by Zapiersteps expect adict` to be returned; you're returning a string.
Zapier should throw a more explicit error here (something like "expected dict, got str), but isn't. Instead, it's calling .copy() on the output, which results in the error you're seeing

There are two options to fix it:
Set a key on the pre-defined output dict (the currently accepted answer)
Manually return a dict: return {'field_name: 'task #' + input_data['row_number'] + 'assigned'}
Either will work here.