My form users are getting the error "not allowed to be completed this post has been flagged" upon hitting the submit button

I've been able to use the form successfully, as has one other person, but I'm being told by others that they're getting an "not allowed to be completed this post has been flagged" error when they hit the submit button.

Do you have any idea why this would happen?

We would like to apologize for any inconvenience this is causing you.

For me to be able to help you, I have tried submitting your form. However, I wasn't able to reproduce the issue your respondents are experiencing and I was able to submit your form without any issues.

We may need to ask for more information from you so we can further investigate the issue.

May we know if the form was submitted through its direct link and not embedded on any website?

Any specific web browser your respondents used when they encountered the issue?

Are they trying to submit the form using their mobile devices?

Any information that you think would help us would be greatly appreciated.

We will wait for your response.

Thank you.