Maybe one of these reasons are why…check out the solution.
Interlock plunger missing
Remove transmission and transfer case, install interlock
Clutch fails to release
Adjust clutch pedal free travel or replace
Gear shift end worn or damaged; binding in housing
Shift plate worn or bent
Shift rods binding in case
Replace, check case for damage
Transmission loose on bell housing
Clutch shaft pilot binding in bushing case or shift housing damaged
Replace pilot bushing, measure pilot end for tolerance, examine housing
And of course the easiest reason might be not enough free play on the clutch.