The error in the given sentence is a subject-verb agreement mistake, where 'IS' should be used instead of 'ARE' because 'parking is singular.
The error in the sentence 'PARKING ARE FOR CUSTOMERS ONLY. ALL OTHER VEHICLES WILL BE TOWED.' is a subject-verb agreement error.
The correct subject-verb combination should be 'PARKING IS FOR CUSTOMERS ONLY' because 'parking' is a singular noun and should be followed by a singular verb.
This sort of error is frequently observed in signs related to campus parking policies or regulations.