What are the possible issues 6 speed trains will not shift into 1st or 2nd gear?

Linkage check vs replacement first would've been ok, however the transmission in your vehicle if the ID matches is known for this. They tend to develop a burr or spur in an internal part that prevents 1 and 2 engagement. Usually removal of trans and install of updated parts to correct. Recommend having the vehicle checked by a qualified technician to verify.

Is this an automatic or manual transmission? If manual, you could have a synchronizer or gear issue mechanically, and/or a clutch wear issue.