Here are some tips that can help you fix this:

• Check AWS WAF request logs (you need to enable this)
• I cannot see from the image what content-type you are sending, that can be a problem also, default is "application/x-www-form-urlencoded", but that maybe is blocked by WAF
• Last one method rules should be checked or some overwriting rule.

I think it will be more helpful if you can share more of WAF configuration.