This is expected because I was assuming things about your data model that were untrue apparently.
First clue - given the new information about the data model, Categories is a linked field (apparently). As such, you cannot filter based on “=”; rather, it likely requires the “FIND” (in field) operator, right?. The added complexity of the linked field could be at issue as well. Lastly, what scripting environment is the API call coming from?
Forum-debugging of API code (at best) is about guidance - we can provide guideposts, but we can’t build it because it would be extremely counter productive without full and complete access to all the resources - ergo, it’s now a project - ergo, it’s now a consulting gig.