i just upgraded to mojave and can’t sign in

You have to have an Administrator username and ID. You may have chosen "Automatic Login" in which case you may have forgotten it. The user name is the name of your Admin Account and the password is something only you know.

This is all assuming you mean the Login Screen at the end of the boot process. If you mean you are getting a acreen with a lock on it, early on then it's a firmware password.

Let us know which it is.

No, you don't have to have a password. I don't either and had this same problem after upgrading. A reboot fixes the issue.

When the computer first boots up from the upgrade, it doesn't accept "enter" as a option without typing anything in the password field first. My password and I assume the OP, are blank. If you reboot the computer, it will accept "enter" without any text in the password field and log you in. I'm not sure what the glitch is.

Well.. the reboot worked for one of my computer's and not the other. Neither one has a password, but I can only login to one of them.