Stratos is correct here. If none of the Areas are able to be discarded, you CANNOT do the "Seek New Horizons" option at all this turn.
I don't know if you're referring to an older version of the rules, but the manual specifically addresses this situation.
Anima Instruction Manual wrote:
(Under Seek new horizons, p.9)
If it is impossible to select an Area without violating one of the above conditions, the new Area is added without any other Areas discarded. This is the only exception to the rules governing the maximum Area limit.
But like so much in these rules this just leaves an unusual situation completely unaddressed. You only check for too many Areas when you do a Seek New Horizons action; and if over, you only remove one area. So if you do it carefully, it seems you can over-stuff the board with Areas.