When I built a selection query in clause mode it wrapped the target info I was looking for in single quotations, not double quotes and it turns red when displayed in SQL mode. In my example I am selecting anyone with initials J (text) in field INITIALS:
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When I replace single quotes with double quotes in SQL mode and then try to return to clause mode I get the message you are getting and the entry in double quotes appears as black text.
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I assume the warning “the expression can’t be edited in clause mode” is letting you know that you can't switch from SQL mode to clause mode as it doesn't recognize the syntax used (double quotes).