Reinstalled Win 10, wont let me activate

It IS tied to your account. It's technically called "Digital Entitlement".

For all intents and purposes you are still tied to the same hardware, so if you are trying to login with a different machine (i.e. new build) that's not going to work.

Microsoft has various rules on when and how you can transfer your DE to another piece of hardware, but in general if you build a new machine or try to put W10 on an existing machine you'll need to buy a new W10 license.