Guild Stash

Hi,
I can't seem to buy the guild stash. I've tried repeatedly in game and on the web browser but the only option is to add more credit? I thought during the sale its 35 credits for the Guild Stash? I have 40 credits so shouldn't that be enough?
is there some kind of bug or is this a trick for us to only get the stash when the special deal is over?

All I want is a guild stash

Please attend to this as soon as you can. Much appreciate the help

Thanks