Hi Ricardo,
Sorry to see you have been struggling with issue gift card in CPOS for 2 months. I have some thoughts and a link that will hopefully help resolve the issue for you.
Thoughts: The error " IssueGiftCardClientRequestHandler received a null GiftCardItem" to me says that the gift card Item ID is not populated in the channel database. However, I will assume you have tried configuring the gift card product a number of different ways so I don't think that is it this time.
While preparing for the Internal Gift Card TechTalk I saw an issue that I consider a product defect. The issue gift card operation IS NOT the same type of operation as the product sale operation and in the past the gift card product did not need to be assorted to the store. In my option it is better to not assort the product because you do not want a cashier to try and 'sell' the gift card product. It seems that sometime recently a conflict between the gift card operations (and the gift card item) and the POS permission 'Sell non-assorted products' was introduced.
So, this may sound odd but check your POS permissions and make sure 'sell non-assorted products' is TRUE for your POS user. Regardless of the gift card item being in or out of the assortment and regardless of the POS user having the Manager privileges set to TRUE. Remember to run the 1060 CDX job after you change any POS permissions.
If that doesn't fix your issue, then the much longer process I can suggest is to go through the presentation that goes with the TechTalk that was given last November. Link:
community.dynamics.com/.../d365-commerce-internal-gift-cards-november-16-2021
In the TechTalk we used a master product with variants because that is needed to sell multiple denomination on ecommerce, but a simple product will work just as well for POS.