If a Credit Memo was created in Aug. (Period is closed now). It was created for wrong Account "Xyz". We want to cancel/Void/Reverse that and create that in New Account. What is the best way to do that? The Reverse JE doesn't work , it says "You are not allowed to post to AP via JE". Any Suggestions ?

Seems to be a customization specific to your environment, no actual system limitations around posting to AR/AP directly from a JE - assuming you mean AR since this is a Credit Memo?

I found this : Yes, not being allowed to post directly to Accounts Payable (AP) or Accounts Receivable (AR) via a journal entry is a standard feature in NetSuite. This restriction is in place to maintain the integrity of the AP and AR subledgers and ensure that transactional details are correctly linked to customers and vendors.

NS allows it out of the box. We do that sometimes. You have a script that must be preventing you from doing so.

@Alex K @Luis you guys are absolutely correct it was a custom script. I got it.

Instead of JE, you could create an invoice to match it against. same JE effect

It is resolved by removing that custom restriction and posting a JE