I'm no tranny mechanic - but I've had a few break on me - Check first the shift linkage. Sometimes these can get out of whack & if you just had it out they may have popped loose.

Second - pull the shifter out & see if you can shift into 1st by reaching in with a big screwdriver. With the clutch pushed in the shift forks should move around freely.

Look for any loose bolts. They can cause binding issues too.

Its possible the used tranny had more use than you thought too -