Hello, are we able to adjust inventory for “non-inventory item for purchase” types?

No, non-inventory items are not stocked items. Items that you always drop ship or other items that you sell or purchase but do not stock is what non-inventory types should be used for. Since NetSuite doesn't stock non-inventory items, you cannot adjust the stock of that item type.

Thanks for the response. I was afraid so. Supply chain wants to adjust to track the stock levels. They may need to manage this outside of NetSuite....

You can convert non-inventory items to inventory items. See page 29 of this PDF https://docs.oracle.com/cloud/latest/netsuitecs_gs/NSIMG/NSIMG.pdf be mindful of GL impact changes going forward