The challenge room is locked but I have full health [duplicate]
Solution 1:
This is a boss challenge room. Indicated by the skull on top of the swords.
To enter these you need to have maximum one full red heart. It does not matter how many blue/black hearts you have. You can also have as many empty red heart containers as you want.
Other options to get in are using items/consumables that open all doors (such as dads key) or bombing your way in if there is a secret room adjacent.