Basic troubleshooting: if the clutch was the last thing you were working on, chances are that's where the problem is. I agree with MikeFoot: maybe you left something out when putting it back together. Pull the clutch basket and see if it frees up.
A human designed the bike, humans assembled the bike, a human took the bike apart, human(s) can put it back together. You can do this. It's all part of this vintage MC deal. You can expect little stuff like this to brighten your day. It's also a great learning opportunity. Just think: You figure this out, and people will be asking you about clutches and stuff.
We're here for you. It WILL run again.
If all else fails, have a beer or two, let it sit until tomorrow, let your mind work on the problem, and in a day or so, you'll get an idea about what might be wrong. Unless this bike is your daily driver...
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.