More often than not, a lookup field returns an array, not a string, and FIND()
only works on strings. You can force-convert the output of any field into a string by concatenating it with another string, including an empty string:
FIND('T003: Symptom', {Rel_lookup_Terms_Type} & "")
A deep dive into field types and what they return—including lookup fields returning collections of those types—can be found here: