Google has a limit of 1000 photos per album. You reached that limit.
I am not sure what are you asking us to do. Google+ provides only limited amount of storage for your photos and it’s limiting the total number of photos and the number of photos per album. Those are Google’s terms and conditions that you have agreed when you signed up with Google+.
We are not working for Google and we cannot alter those rules for you.
You can remove some pictures to free your album, move your pictures to another album, or even raise that issue with Google itself.