I upgraded to the new receivables 9.0A. I have a bunch of customers showing "inactive", what does that mean? I also can't post inactive customers and getting errors. These "inactive" customers have balances.

Someone ran "Mark Inactive customers".

Use the SmartFinder to identify those that were marked inactive and make them active again.

I've had a few clients where this cropped up. The customers were already marked inactive in 8.1A, but it didn't cause any complications. Upon upgrading to AR 9 and OE 8.1C (forget which one actually incorporated this change), you are no longer able to create orders/invoices in OE for a customer marked as inactive.

If there are a lot of customers marked inactive that shouldn't be, you can use the smart finder to export customers (using a filter based on outstanding balance or last invoice date) to excel. Then import this list back in and default the "Active?" field to "true". Your Adagio Consultant can help you with this.