Lets say you want to pick up Chase McLaughlin - you click on the "+" to the left of his name:
You see how I have the option to "add Chase McLaughlin but don't drop anyone"? That's because I have an empty spot on my roster so I could pick chase up right now. You may not, in which case you might have to drop someone - just pick your worst or most easily replacable to drop if you have to do that.
Then do the same thing for your DEF (Defense).
Let me know if this doesn't help.

You see how I have the option to "add Chase McLaughlin but don't drop anyone"? That's because I have an empty spot on my roster so I could pick chase up right now. You may not, in which case you might have to drop someone - just pick your worst or most easily replacable to drop if you have to do that.
Then do the same thing for your DEF (Defense).
Let me know if this doesn't help.
