In addition to Kglad's advice, I have experienced this problem. The cause, it turned out, was an old log-on that Adobe had not deleted when I changed it. I had also set my browser to remember the credentials and it kept logging me in with the old password. I still haven't been able to get Adobe to sort it from their end, but the solution was to log out, then log back in using the most current password. Also note that the Exchange needs a LOT of work. There are installation glitches, and if this is about Dreamweaver extensions, you will not be able to install anything if your hosen language in the CC App is English International. There are a few wee bugs to be fixed
Sorry to hear about this. We're aware of an issue where some users couldn't acquire/install products on Exchange CC. We are rolling out a fix on October 26. Until then, you can sign out and then sign back in to Adobe Exchange to try and fix the problem. Please let us know how you get on.