I haven't played the game yet, but I noticed after checking to see the new version (offline installer) that under Serial Key it says ran out of keys?

Keys for what?

How can you run out of a digital item?

its a surge key and only for people that got the game before June last year

https://www.gog.com/news/special_offer_bbuy_mechwarrior_5_and_get_the_surge_as_a_giftb

p.s, you can buy it from gog if you want
https://www.gog.com/en/game/the_surge