What is it, how do i get it, and where do i get it?

i have everything else but that to build more space

It means you don't have an active pet that meets the building requirements. Look under the Lists of Requirements section, you need a pet that meets those requirements in order to build.

then what pet do i need? and how do i see the list of requirments? all it says is the insufficient pet no matter what i choose?

do you mean this?

if so, i have no clue what it means...

If you look on your pet page, the stats are given there.

It tells you your pet's strength and constitution and intuition and so forth. You need a pet with a constitution stat higher than 60.

I recommend going to Pet Trades and clicking on the + button, then searching for Constitution = 60, then buying whatever's cheapest.