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Cant save data in workflow even though visible in droplist

I am trying to save data from a popup form and 1 field is sourced from 1 to many database. This field is on a droplist which works ok but on the create new thingy workflow the ‘droplist’ item is greyed out and says ‘incompatible type’. I recreated the 2 tables in another app and it works ok, what could be my issue?