Runtime error 91 in POS or PO

Error in POS recall -

Means item deleted or discount type assigned which no longer exists.

Does the following query return any results?

select quantitydiscountid, * from item where quantitydiscountid not in (select id from quantitydiscount) and quantitydiscountid <> 0 3.

If so, do any of the items returned in the query above also list with:

select itemid, * from transactionholdentry

What these results indicate is that you have items, assigned to the transactions on hold, where the assigned discount no longer exists; and this may be causing the issue.
Please go into the item(s) and either remove the discount or change the discount to one that is currently available. You may also try adding a valid discount, the remove it.

Error in PO commit -

Means items likely does not match original PO entry such as assigned to a supplier which has been removed. Check Supplier tab and delete and re-add correct listing for supplier assigned in PO.

Error can also be caused by triggers firing during commit such as AutoGen, Supplier tab, disable second entry if running 2.0.0155 or higher.