I just picked up 2021 800 XC with 77-Miles on it the other day and noticed that from time to time I'm unable to shift it.  It seems to happen more often when I'm in reverse and trying to shift after backing up.  I am stepping on the foot brake that doesn't change anything.

[2015 Z Force 800 53/Trail EPS]

If it shifts after you shut it off then it's likely the idler bearing that the belt rides on at idle is starting to seize. When this happens it tries to rotate the belt applying pressure on the secondary and thus the transmission...and that makes it hard to slide gears around. I very common problem. If so, just replace it.

Mine does the same thing but only when it's cold, meaning I let the machine warm up for a bit then it works fine. Or like NMK said it could be the one way....
sol...

Other then the above mentioned

Do you let it idle down before shifting

I have a new belt on my uf1000 an if it ain't idled right down till the belt wears in its shit to shift

Also can you adjust the linkage always an issue in the first few hundred miles

It does it hot or cold.  I am letting it idle down prior to shifting or attempting to shift.  It's not all the time, but it's 90% of the time, it's only when trying to shift out of reverse.  Turning it off doesn't help at all, rocking the machine seems to help slightly.  I'll try the idler bearing and see if that helps.

I've had to rock my 800 XC occasionally to be able to shift easily since it was new.