you've got one of the uncapturable monsters on your team...that's the standard error...
problem with these monsters is that you can't tell the difference whether they are "normal" or uncapturable...
usually, the full 3 stars, or any monsters with last star empty, are uncapturable.
you just have to try to remove them one by one....best if you remove all monsters, leave 1 in there, and try to click "online"...if you manage to get in, then that monster is "good", you can go back to the team and add next monster etc...
I usually have 1 full team of "normal" monsters, and the rest of the teams will be full of uncapturables. when you want to go to the "online" part, you just have to click the normal monster team tab (to activate), and then click the "online"
enjoy