I tried using a Ref in a user setting and it worked for me. Perhaps I do not fully understand what you’re doing and so cannot see what’s wrong. My understanding is:
You have a user setting named ManyChat that is of type Ref with a source table of ManyChat Fields.
The initial value of the ManyChat user setting is
FBFoodOrdering, which is presumably the key column value of an existing row in the ManyChat Fields table.
You attempted to set the ID user setting to the value of the ManyChat user setting using the expression,
[ManyChat], which appears to test OK.
You then attempted to set the same ID user setting to the result of the expression,
[ManyChat].[API Name], which produces an error.
Is that correct?
As I wrote this reply, I noticed this:
Note that at least one user setting must have Show? set to ON in order for user settings to work.
Note, too, that user settings have no value until the user accesses and saves them at least once by going to the main menu, choosing Settings, and saving the resulting form. This does not appear to be mentioned in the help doc…
It might also help if you could briefly explain what you’re trying to accomplish here.