"Crock up" is a phrase that doesn't have a large selection of synonyms. However, there are some phrases and words that can be used interchangeably or in place of "crock up." For example, "mistake" is a good synonym to use if the context is about someone making an error. "Mess up" can be used if the context is about someone making a mess of something. "Blunder" and "slip-up" are also good synonyms to use when conveying the idea of making a mistake. Overall, while "crock up" may not have many direct synonyms, there are still several phrases and words that can be used in its place.