My foreman 450es shift sticks also when I'm at a dead stop. Common problem with them. You might try rocking it back and forth while trying to shift. This works most of the time. I sometimes have to get the shifter tool out and manually shift the gears. Usually if I do that once its good for awhile.
However if you are having trouble shifting while moving none of this will help.
Hope you get it figured out.
They will also do this when the battery has low voltage and if the idle is set too high. I have two foreman ES, '01 & '02. One, the idle was set too high, and once properly adjusted, worked fine. The other was due to an older battery. Once I replaced the battery, the problem went away. Still have to rock them back & forth some times also.
I have an 05 Foreman ES also. My battery had weakened and mine would do that and could not get it to shift into neutral so I could start it. But if you pull both brake levers, or use foot brake and pull the front brake lever at the same time it will start no matter what gear you are in. Once mine started it would shift fine. So I just left it that way until the battery was so bad it wouldn't hold a charge at all. Eventually replaced the battery and issue went away.
There is an electrical connection on the back of the transmission that can come undone. When this happens it will not shift into reverse. Lay on the ground behind the bike and look up past the rearend and you should see it.