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

Error: The Managed Metadata Service or Connection is currently not available. The Application Pool or Managed Metadata Web Service may not have been started. Please Contact your Administrator.

Steps to troubleshoot:

• Check Managed Metadata Service is Running from SharePoint Central Administration site >> Manage services on server, Click on “Start” link next to “Managed metadata service”
• Start MMS Service application pool from IIS 
• Check IUSR is added in Anonymous Authentication in IIS: Open IIS, click on your Server Name in left pane. Now open Authentication widget from the middle pane, Right Click on Anonymous Authentication and select Edit, Click on Specific User radio button and click on Set Button. Type IUSR in user name field and then click on OK.
• Check the permissions on Managed Metadata Service Application to your Web application’s app pool account: Go to SharePoint Central Administration >> Application Management  >> Service Applications >> Highlight Managed Metadata Service  >> Click on Permissions button from the ribbon >> Add the application pool account of your web application and provide at least “Read Access to Term Store”
• Managed Metadata Service is associated with your web application?
  Go to Central Administration >> Application Management >> Configure service application associations >> Select your web application >> Check the Application Proxy Group and make sure that the Managed Metadata Service is listed there. 
• Recreate MMS Service application with an Existing database: Here is how: Delete the existing MMS service application without deleting the MMS database, Create a new MMS service application, Go to the properties of the new Managed Metadata service application, point to the original database 
• Check the URL log. If you find something like “Requested registry access is not allowed” (Event viewer ID 6588), then it’s a problem of Application pool Service account gets access denied. Use: https://technet.microsoft.com/en-us/library/ee513047(v=office.14).aspx to resolve. 
• Last but not least, Perform IISReset!