I am experiencing the same issue as Dave - I can take two IDs that work individually, but after throwing them in an array separated by a comma (with or without a space after the comma) I get the same error. About to break this out into a direct API call to Monday, but sure would be nice if it worked in the Zap step properly.
Hello! I stumbled upon my search to a solution to this problem over a year later! But, i found a solution that I wanted to post here in case anyone needs one. Use the Looping by Zapier to run through each iteam as an iteration. I had the best success adding them as text and separating them by a comma. I have attached pitures for you to reference.