Can I use a hair thickener spray after 2 weeks after FUE Hair Transplant? The grafts should be fixed after 10 days I suggest.

By 9 days the grafts are completely incorporated into the scalp,so yes you can safely use a thickening product now.

It is best to ask your surgeon about post operative care as each doctor has their unique regimen.  In general, there should be no significant issue with using products after 2 weeks.

After two weeks using hair fibers/thickeners is generally considered fine to do. I'd check with your doctor first though, in case there are issues specific to your case that may alter that waiting period.