Possible cause #1 The AR Category number has been changed
On the Point of Sale window, enter AR in the product code field and then click on the Inventory search icon. When the Inventory window opens, make sure the Category number for AR is 003,
Changing the inventory category number,
From the Point of Sale window, click on the Inventory search icon,
Find and select the AR item,
In the Category number field, make sure that the number is 003, if not change it to 003,
Click Save.

Possible cause #2 The AR has been already opened for this customer in a previous estimate
You need to take a look at your estimates, find where it has been opened, and close it.