The 500 errors are not very helpful. Almost anything being off will throw them. Double check id numbers. Make sure the column type is “people” NOT “person”. Also, looks like you are using at least 2 different types of quote characters.
It seems to me like your JSON formatting might be off, or the values you are trying to send aren’t matching your account data. I was stuck with the 500 server error as well until I noticed the User ID wasn’t matching a User ID in my account. Some tips and pointers to make sure you get this right:
Make sure your Board ID, Item ID and User ID is correct;
Make sure your JSON string is escaped correctly;
Make sure that you are sending the mutation wrapped in a query if you are not on the try-it-yourself page;
You will have to change all of the numbers to what works in YOUR account. Also, I just noticed that what I pasted is NOT what is showing in the post… the “internal” ‘\’ marks and type of quotation marks are important… try this (with the numbers replaced):