Account of the last owner won't go away ?

That lingering account is likely tied to your uncle’s Microsoft account still linked to the device license. To fully remove it from the lock screen:

Go to Settings > Accounts > Email & accounts, and remove any old Microsoft accounts listed there.
Then go to Settings > System > About > Advanced system settings > Computer Name and click Change, to update the PC name if needed.
For the license, sign in with your own Microsoft account under Settings > Accounts > Your Info, then go to Activation and link the digital license to your account.
If all else fails, a clean reinstall will fully wipe all traces—but now you know what’s left behind.