@Mr3051 let me be one of the first to welcome you to the OfferUp Community forum,
Mr3051 Error; R37KPRMM or messages saying your about to Violate OfferUp guidelines
Usually mean you are doing something against OfferUp guidelines, which is fixable! Just make sure you aren't sending out personal information or sending people the same exact message as it may be flagged as spam. or perhaps a mislabeled server error if you believe you've done nothing wrong.
If you're in good standing the issue will resolve itself. You should be able to message after 24/48 hours, if not I would recommend contact OfferUp customer care from the link below. I have also added the OfferUp selling app guidelines below as well. 🇬🇧😎🔚