I just bought Fallout 1 and I am unable to install it. In the GOG Galaxy client, the install button is greyed out, so I went to the site via browser, but when I click the files to manually download it just goes to a white screen with text saying:

{
"error": "invalid_licence",
"error_description": "User #46989215992712526 does not have the license for product #1440148836."
}

Anyone know what to do about this?

I've seen a couple of other reports about "invalid licence" error. Most probably it's some GOG's database malfunction that they have to fix.

a little bit waiting then - go to gog.com/ account update
http://www.gog.com/user/refresh
or
http://www.gog.com/account/refresh
(one is right another one is outdated probably, somemethign like that).
Reopen games list on account (after redirecting to acount after that links go to wishlist and bak ti library) and\or restart GOG Galaxy.