Ue4 using box collision

ue4 box collision block you if ur lvl is lower than 20 and overlap u if ur lvl more than or equal to 20

When the player levels up, check their level and set the desired collision state of the component. How to set it up depends on what you have atm - of what we know nothing.

but if there is more than one player and there is one with lower than 20 and one with 20 or more if i set the collision not enabled if the one with lower than 20 will be able to enter what should i dio to only block the one with lower than 20 should use on event begging overlap

You can change physical preset on the pawn itself. You need to have two collisions, a bigger one to get overlap event and check the level if the player level is okay allow it to pass through for the smaller collision
It might help you How to make one-way collision


but how can i make the smaller one block

Add collision box or capsule and tweak its size accordingly

You have some collision on pawn, just add the bigger one to use it only for level check