I'm playing with Arsenal (the real team, not the default one), and I can't buy players, I can only thade them. Why is that?

Probably you don't have enough money yet.

But I can't even try to buy any player. Looks like this option is not active.

maybe you have to check the option when you start new ML

or maybe your team doesnt have enough space for new players?
i just guess because i dont know if there is a limit of the number of players in the team. :p