Try releasing and then pulling the clutch in and then put it in gear.

The transmission gears have cogs on them with big square shoulders.  If the bike is stopped, and the clutch is pulled in and the transmission shafts are stopped.....the cogs may not be aligned in a way that allows them to mesh properly.  You either have to let out the clutch and pull it back in and shift before the transmission shaft comes to rest.....or you will have to rock the bike forward and backward a bit to allow the cogs to mesh.  This is completely normal.....and becomes second nature after a while.

If pull in the clutch and wait too long before shifting into first.....you can expect that it will not shift into first most of the time.

Whatever you do,.. don't just keep mashing the lever to force it... that's how all these bent shift rods happen...
When it's aligned,... it will snick into gear with very little effort... when it's not aligned, it ain't gonna' go...
Brains not brawn... ...

If you sit, in neutral, with the clutch pulled in, all the cogs in the tranny stop spinning. If they stop so that the shifter can drop it into first, great, but if teeth are lined up on teeth, its not goin,, If youre parked, in neutral, let the clutch out,, if you need to GO, right NOW, pull the clutch & step on the shifter,,
remember, aim for a " I wont get sandwiched" spot & stay in 1st gear if you expect anyone to come up behind you. Tap the brake pedal, squeeze brake lever, whatever, pulse that brake lite, move the bike some, whatever, just dont let anyone fly up on you.