It means that you have too many tanks in storage. You need to retire some of them in the Factory or use some of them to strengthen your other tanks. A similar message appears if you are within 20 equipment of your equipment storage limit.
Alternatively, you can purchase Headquarter Expansion items to increase the number of tanks and equipment you can hold. They are 98 Diamonds each, but there is a very good limited-time offer right now for 10x Headquarter Expansion items for 300 Diamonds during the Case White event. It would be a good idea to purchase some of those if you intend to collect a lot of tanks. Right now there is a hard limit of 1050 Tank Storage and 4200 Equipment Storage, but that will be increased during the next update.
Ah, I see. Thanks for the info.