EF04 Error: Invalid Accounting Flexfield. Check your cross-validation rules and segment values.

Dear EBS Experts,

We would like to seek your expertise on the Error we get when posting the journals having EF04 Error.

1. What would be the cause of Invalid Accounting Flexfield,
2. How do we check the cross-validation rules and segment values?
3. What would be the cause of an error of" the date not being a business day."?
4. How do we resolve this: "Value ABC123 for the flexfield segment Local Sub Account does not exist in the value set" In the first testing we used 000000 instead of ABC123 and the error is kind of fixed, is there another solution?

Appreciate your help.