Recently I have come across an issue, where the CRM users complain that they cannot add Write-in products to the Quotes and Orders.
It was a CRM 2016, which was upgraded from CRM 2011. In this case, we need to check the forms and the subgrids.
The write-in products can be added only to the grid which has the CRM default name, if you have there something else, then change it to the correct one.
For quotes - quotedetailsGrid
For orders - salesorderdetailsGrid
Also note that for Orders, you can only add new product if the pricing is not locked. Click on Use current pricing if you have the pricing lock on.
Happy CRM day,