He couldn't sign a new contract with Barcelona, even if it was for free (which I think it'd be ridiculous if he had to sign it anyway). Barcelona without Messi is already spending more money on salary than La Liga allows so they can't make any new contracts until they solve this problem. Only if they had dismissed 4 key players they'd be able to sign Messi, which the president didn't think was a good idea. (That's the justification I've found online, these informations might not be 100% accurate).