Using Sum function on 2 referring pages

I am trying to sum to different cells on 2 different Smart Sheets and I I keep on getting a #UNPARSEABLE error. It works fine when it references only 1 sheet but I need to sum probably about 20+ sheets when I am done. Just testing with 2 right now.

Any help is appreciated

=SUM({AAA Pet Services - Sign Ups Range 1}{AAA Pet Services - Sign Ups Option 2 Range 1})

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