Minecraft – Why can’t I play online after changing the profile name

While you can play Minecraft offline, you still need to have logged in on a Minecraft account. This is used to verify that you actually own the game. If you are not logged in and don't have an internet connection, you won't be able to log in.

You can install Minecraft from a USB drive but you will need internet access, at least temporarily, to log in once. Once you've done that, you'll be able to play offline.

It's caused by remaining logged in after changing your name. Not everything is updated.

You need to open the Minecraft launcher, log out, and log back in again (i.e. you need to enter your user and password again). This should fix it.