I've never gotten that message. Maybe cut some loser (just about every team has one) to free up one slot and see if that keeps the game from complaining?
Yeah, this confused me too. What you can do is look at what plays they don't know (if it's a guy you just drafted, they won't know any of them) and choose a play as the focus for your practice. This will coach the unlearned in the play, and, typically, that will include the goal player.
To see what an individual's playbook knowledge is:
-View you playbook
-Each play will have Unlearned/Learned/Mastered. I'm assuming that's the average knowledge of the play
-You can select a play to get an individual player's knowledge of the play. Your whole roster will be shown, you can filter by position.
Then you can make a note of what plays you want to run in practice. If they don't show up in practice, make a point of using those plays heavily in the preseason games until they are learned.
Like when I had to train up Dumervil after picking him up in a trade, I chose to focus on defensive plays he didn't know.
One thing to keep in mind, the other players aren't just sitting around when you're focusing on someone in training camp. You'll practice 5 plays and many times you'll get some more knowledge for other guys involved. Say your C and LG didn't know the plays that were run - they could end up learning them, even though the focus was on the QB.
Also, when preseason games come around, I like to call only unlearned plays (choose plays by playbook knowledge, then choose unlearned) as this will help them learn plays in games that don't matter. Keep in mind that the backups will come in at some point, so you might want to repeat the process so they can learn the plays also. Practices tend to focus on the starters first (just like in real life, the starters get most of the reps) so the backups often end up falling behind.
There's also a trick you can use where you can quit the game before time expires and players will keep the knowledge they had in the quit game. I don't use this, but it's an option, especially if you get desperate and don't want to fail the goal.
Scouting the next draft class will happen during the regular season. You'll get "College Scouting" as an item on your clipboard and you can hit X on it and then scout Big, Medium, or Small schools by position (like you could do Big School QBs or Small School LBs, etc).