I know my correct Apple ID password but it won't work on my main computer. Why is that?

Hi KeikoCat,

Have you recently changed your Apple ID password? If you have, you may need to update your sign-in information with iCloud services on your Mac.

This article will explain in more detail: What to do after you change your Apple ID or password

Mac

If you're using macOS Catalina, choose Apple menu  > System Preferences, click Apple ID, then click iCloud. If you're using macOS Mojave or earlier, choose Apple menu  > System Preferences, then click iCloud.

• If you're signed out, enter your updated Apple ID and password to sign in. The data in iCloud will automatically appear on your device after you sign in.

• If you're signed in with your previous Apple ID:

3. Click Sign Out. Depending on what iCloud options are turned on, you'll be asked if you want to make copies or delete iCloud data (Contacts, Calendar, Reminders, Bookmarks, Photo Stream) from your Mac. Your data will remain in iCloud and will be updated on your device when you sign in to iCloud again.

4. Enter your updated Apple ID and password to sign in. 

If you're still unable to use your Apple ID on your main computer, a few additional details will help us cater our suggestions:

• Are you able to use your Apple ID on other devices?
• What type of computer and operating system are you using? (e.g. iMac on macOS 10.15.1)
• Where are you attempting to log in? (e.g. iCloud, Messages, etc.)
• What happens when you try? If you get an error message, please post it. The exact terminology or code will help us identify the issue.

Thank you for being part of the Apple Support Communities.

Warmest Regards.