Hi, it could be two things according to my limited knowledge.
1. The gearbox alignement can be adjusted with some bolts. Others will chip in on how to check / do this.
2. The plastic bush on the end of the gearstick is defective, broken. I had this and could not select 2nd / 4th gears. I had to replace the gearstick. It's not an expensive part.
Hopefully this will point you in the right direction.
R