Your clutch is not releasing completely. The trans cannot shift when the input shaft is still turning. There is a very small gap between the clutch, disc, and flywheel that is hard to test for. The inspection cover on the bottom of the bell housing is there for that. You remove the cover, have someone press the clutch pedal down, and you test with a feeler gauge for the release gap. To correct this you need to get all the clutch linkage adjusted correctly and it doesn't matter if it is all new or old. When you press down on the clutch pedal, the linkage geometry is designed to make it easy to compress the pressure plate springs. So a foot of pedal travel translates into a fraction of an inch at the pressure plate and disc. Have you adjusted the linkage? The bottom rod is the adjustable part. You want an inch of free play at the top of the pedal, while getting total release at the pressure plate. The clutch fork ball stud is a major adjustment point, and there used to be three different lengths. Now all I see are the tall and short ones, plus an adjustable one. Same with the throwout bearing, there are tall and short ones. A common issue is the clutch fork is hitting the back of the fork opening in the bellhousing, limiting the travel. There are also adjustable spherical ball end upper and lower linkage rods. I am using those because I could not get my geometry correct and they helped a lot. Start at the lower linkage rod. Adjust it so the clutch releases totally. You should not have the pedal hitting the floor, it should be completely released about 3/4 of the way down. Then see if you still have the right free play at the pedal.