For the past couple of macOS major updates, it appears that the Remote Management/Screen Sharing service needs a reset because something went wrong during the upgrade process.
If you're trying to connect to a Mac and Screens seems stuck at Downloading... or you receive an incorrect username or password error, you may need to perform some steps.
Note: these instructions should be performed while you have physical access to your Mac or you may lose access and not able to reconnect.
Reset required services
When this happens, try to switch from Remote Management to Screen Sharing (or vice-versa).
You can then switch back to Remote Management afterwards but make sure that Observe and Control are enabled under Options...
... and that the Mac user is allowed to connect via the Remote Management service.
If Observe and Control are already checked you can uncheck both options and re-check them back in order to reset those as well.
If the macOS firewall is enabled, try to turn if off and then back on.
Related Apple Article: If you can’t share Mac computer screens.