@Glorfindel is correct - you had an answer flagged as spam.
You linked to a survey-site offering "Free" Robux for completing surveys.
While you may not have known - those types of sites are typically very shady (if not outright scams), and should be avoided at all costs. Typically these sites are trying to steal accounts by offering ...
Looking at your reputation history, I can't help but conclude that you must have had a post flagged as spam or rude/abusive; if six users (or a ♦ moderator) flag a post as such, you'll lose 100 reputation. Your reputation tab should show which post it was. (Other ordinary users like me can't see those.)