Question: Stop command doesn't work

I have written a program and I would like to embed a check near the beginning to see if input data was entered correctly.  I have written an "if statement" to check on this and I would like the program to stop execution at this point if the "if statement" criteria are met.   I get an error saying that this command is disabled.   I have tried "stop", "end" and "quit," as I understand that they are all the same.

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