Have you fully synced the wallet you are using? that would be the first thing to check
Yes, because web wallets require you to sync before letting you spend
/s
Don't signature spam in a section where people actually need help.
To OP - Have you tried contacting greenaddress's support? It could be that their backend is messed up somewhere, so try emailing them. Their email is
info@greenaddress.it (see Support tab on their site). Also, try switching browsers, as the button might use javascript or code that isn't supported by your browser.
If this solved your problem, and your actually serious about paying, the address on my profile will work. Thanks