I bought the players handbook but can't choose rhe Assassin subclass.

As in the title, I've bought the digital book into this account but for some reason I can't put it on my level 4 rogue as a subclass. 

Which version of the Player's Handbook did you purchase? There are two: 5e/2014 and 5.5e/2024. You'd need to be using the version of the Rogue class that matches the version of the book you purchased.

If you just purchased it recently, it's also a good idea to go here and see if it says your account entitlements are in sync. If they're not, click the button there to sync them, and then try again.

I did find it eventually after some deeper research outside of the heat. I did buy the 2014 one just didn't know I had to choose the legacy rogue. Thanks anyway.