ok cool dude do you have any more problems i could help you with. and till you can make room for more monsters in the mean time you could sacrifice them to traits and other monsters
to get rid of monsters you dont need in your inventory you will need to unlock the sell function. you do this by getting enough gold to upgrade the merchant place found in the campfire. it should be the first 1 you see. then after you get the upgrade. you should see a sell button on the top left corner in you inventory
@Nidht to lvl up past 39 it will require monster of higher rarity. common is grey, rare is green, epic is dark blue, yellow is legend, and light blue is master. if your using all commons and your stuck at 39 that means you need to use rare monsters to push that monster past that threshold.