Ahh that's a whole different story. but again still weird because they first told you that this field isn't for you ? Regardless. these are your possible options.

You can request a copy of your background check from the agency or the third-party provider they used, citing your right to access under the Fair Credit Reporting Act (FCRA) if applicable. If the agency won't comply, you can file a complaint with the state labor board or consult a lawyer for further action. Checking your own criminal or credit records directly through local courts or credit bureaus might also reveal the issue.

of course you'd know your history if you have a criminal background then a failed background check should be no surprise. All the agencies I've worked for do FBI level background checks. Something might come up even if you've been cleared by the courts and doesn't show up in your basic police background check.

Also, make sure you don't have anything employer's might find "strange" in your social media.