For the past couple of macOS major updates, the Remote Management/Screen Sharing service may need a reset if something went wrong during the upgrade process.
If you're trying to connect to a Mac you were able to connect to before upgrading macOS and Screens seems stuck at Downloading... or you receive an incorrect username or password error, you may need to perform the following 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.
1. Reset required services
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...
If Observe and Control are already checked you can uncheck both options and re-check them back in order to reset those as well.
Also, ensure that the Mac user (or all users) is(are) allowed to connect via the Remote Management or Screen Sharing service.
If Remote Login is also enabled, try to disable and re-enable the service as well, also ensuring that the Mac user (or all users) is(are) allowed to connect via the Remote Login service.
2. Reset Firewall
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.