I think that what the phone is saying is right. Since your internal storage is almost full, you might not be able to store stuff on the SD Card or open some apps. Trying freeing up your Internal Storage.
Sort through your room. You should be able to find some things that you can throw out or donate. If you find anything you haven't worn or used in the...
It might be glitched. Try reloading it or exiting out of the app and rejoin it and try again. Also check to make sure that your storage is not taken...
The question does not have enough detail for a proper answer. To answer it, we would need to know what operating system you are using, Windows, Linux, Unix, Apple, Etc?...