If you are using nothing but a custom group, there is no way to set it to anything but level 1-54.
To set the minimum level of the mission, you either need to add an enemy group of the minimum level, or add a hostage of the minimum level (can either be an ally or foe). You need to add a group that is of the minimum level that you want.
Let me never fall into the vulgar mistake of dreaming that I am persecuted whenever I am contradicted.
~Ralph Waldo Emerson
"I was just the one with the most unsolicited sombrero." - Traegus
Put a critter in the mission which only exists in the level range you want. It can be anywhere.
For example, if you want the mission to be level 30+, find an enemy that exists in the 30+ range only. Then set it as an enemy, a boss fight, a hostage, an ally, whatever.
Please note that it works both ways. If you choose an enemy that is 30-40, not only will the players be auto-SKed to 30, they'll be auto-exemped to 40 if they're above.
@Roderick
Strongly suggest you go through these tutorials.
Please don't respond and say raise the level of an ally or eb to the desired minimum level. TELL ME HOW TO DO IT. Literally walk me through which parts of the UI to click on; hold my hand. I have yet to receive any helpful bit of information in or out of the game. How hard can it be to do this?