You probably reached or exceeded the keyword limit on your account - don't worry, we got you covered!
If you want to add another project keyword, then you need to either delete the old one(s), upgrade your current subscription, or buy extra keywords to increase your limit.
To do so, go to your Account Settings
and select Your Subscription and account.
You'll be able to upgrade your limits over there, feel free to reach out to your Account Manager if you need any assistance with this :)