To increase the Level Cap in Pokemon Masters you have to do a couple of things. The first is to progress far enough into the story to have recieved your first badge and spoken with Professor Bellis. Once you have you will have to complete the Special Training Mission: Battle Techniques in the Training Area. Then you will need to complete the mission “Reach Past Your Limits”. On this mission Bellis will pit you against a powerful trainer. If you win the fight you will be given the unlock level cap option in Trinnia’s (Green NPC on right side) menu in the Pokemon Center.
Increase Level Cap in Pokemon Masters
The initial level cap is 30, but after you complete this mission you will be able to go past that.
You will also need to go to the training area to collect special items required for unlocking the level caps on specific sync pairs. Each Sync Pair will require different items to increase the level cap, but they can basically be gotten pretty quickly if you do the training missions. The higher you try to unlock the more items you’ll need and it’ll be harder to get them.
Once you do your first level cap unlock you will see that your characters can go up from level 30 to level 45. After that you will see that the next level upgrade will be to level 60.
If you’re trying to do this prior to speaking with Professor Bellis you will not see this option in the Training Menu. You’ll need to wait until you progress far enough into the game to get this option to increase the level cap in Pokemon Masters.