1.

Please make sure the Managed Metadata Service is associated to the Web Application where this error occurs
CA > Application Management > Configure service associations > click custom > check the box Managed Metadata Service > OK
If error still occurs or the box is already checked go to 2.

2.

You pretty much just miss the permissions for web application’s app pool ID to access Managed Metadata service application. All you have to do is to add the account used by the web app’s application pool into the ‘connection permissions’ as below. Try not to mess around with SQL user security/DB mappings as suggested by other posts:

2.1. Go to SharePoint Central Administration Site –> Application Management –> [Service Applications] –> Manage service applications
2.2. Highlight the Managed Metadata Service that your web application is associated with (Do not click on the link, just click somewhere else on that row to highlight it)
2.3. Click on Permissions button in the ribbon
2.4. Add the application pool account used by your web application and give it at least ‘Read Access to Term Store’
2.5. click OK