For TLC to be able to evaluate an action, the first occurrence of a primed variable should be of the form
x' = .... or x' \in ....
Later occurrences refer to the value assigned to that variable by this initial "assignment".
In your spec, the first occurrence of carts' appears in the expression carts'[c], and TLC attempts to retrieve the value of carts from the (partly constructed) successor state, but doesn't find a variable of that name. Hence the error message.
Hope this helps,
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