Connecting to a Mac
This error means that the username and/or password you used to establish the Screen Sharing connection to a Mac are incorrect.
This could also mean that your are not using the proper Mac user credentials. To learn how to retrieve your Mac username, please refer to this article: Authentication with Screens
Once you have the correct Mac username, update your saved screen settings and re-type your password (if it was saved already) in order to make sure that it is correct:
Screens for iOS:
Screens for macOS:
Also, ensure that your Mac user (or all users) is/are allowed to connect via the Remote Management service by consulting this article (look for Allow access for:)
You will find more details in this article: Configuring macOS for Remote Access
Remote Desktop/ARD preventing connections
If Remote Desktop is currently running on the Mac you're trying to connect to, it will deny any attempts to connect via Screen Sharing (Remote Management).
Here's an excerpt from a log file on Console.app:
09:45:25.586295 -0700 screensharingd Do not allow access since Remote Desktop is open
Close Remote Desktop and try to connect again.
Another more permanent solution is to configure Remote Desktop to allow incoming connections in the app preferences:
It is also a known issue that ARD may cause connection issues in some circumstances. The solution for this is to re-install ARD, as mentioned in this thread:
The latest of Remote Desktop is available here.
Connecting to another computer type
On Linux, Raspberry Pi or Windows, this means that the VNC password is incorrect.
Refer to the initial computer configuration articles for more information: