Fix SharePoint Error - The Managed Metadata Service or Connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started

Sharepoint Administrators face some unexpected errors while setting up the FARM, some have a standard known fix, but some do not. Here is an annoying error which you may face with the "Managed Metadata Service".

The Managed Metadata Service or Connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started.

1. First things first, check if the Managed Metadata Service is running (SharePoint Central Administration --> Manage services on server --> select Server --> start Managed Metadata Web Service).
2. Navigate to IIS Server and check whether the Application pool for the MMD Service is started and running.
3. Make sure the App pool account password for MMD service is not changed, if yes, then update the new password.
4. From IIS, ensure IUSR is added in Anonymous Authentication.
5. Make sure the SharePoint Web Application App pool account has minimum read Access to Term Store. You can fix this from Central Admin.
6. Navigate to SQL server, rename the database "Managed Metadata Service" to any temporary name and set it back to the original, this works in some cases. The database name may vary depending on what the admin set it during FARM setup.
7. Restrart the SQL Server, this works sometimes.
8. If you see this error in the ULS logs "Failed to get term store for proxy 'Managed Metadata Service Application Proxy'. Exception: System.Security.SecurityException: Requested registry access is not allowed", this happens due to lack of permission in registry key for WSS_WPG and WSS_ADMIN_WPG.
   Execute the command below from SharePoint Management Sheel (run as administrator) to provide access and reboot the server.

   Initialize-SPResourceSecurity

   Refresh the Term Store Management page and it should display as expected.
9. Try running the Product Configuration Wizard, it may auto fix the problem for you.

   PSConfig.exe -cmd secureresources

10. And do not forget an IISRESET if you make any changes to the IIS settings.
11. Reboot the respective SharePoint server, this helps many times.
12. If you are unlucky and nothing works, you may have to create a new Managed Metadata Service.
13. Last resort, reach out to Microsoft support.