Error loading navigation: TermSet not found
This is common scenario to move one application from one SharePoint Farm to another SharePoint Farm, in this process after restoring the database to the new SharePoint Farm following error message came at the top of the site, Error loading navigation: TermSet not found.

Solution:
There are couple of reasons, which could cause this issue. Please check below points.
- In Central Administration, Manage Service Application and make sure that Managed Metadata Service and Managed Metadata Service Connection are started.

- Click on the line of the Managed Metadata Service Connection ( not on name ) and click on Properties

Make sure that “This service application is the default storage location for column specific term sets.” Is checked. If it is not checked then check it and click OK Button.

Now refresh your main site, to see the issue is fixed. If the issue is still there then proceed to next step
- Navigate to Site Settings of your main site (i.e. http://system/_layouts/15/settings.aspx) and Click on Navigation

Now click on Create Term Set button, if it shows success message “Successfully created Term Set” then refresh the site and issue is resolved.
If you are getting error “Failed to create term set: A default managed metadata service connection hasn’t been specified.” as shown below:

or “A default managed metadata service connection hasn’t been specified.”
Then proceed to next step.
- Go to Central Administration, Configure service application associations and then click on default link as shown in pictures below:


Make sure that the Metadata Service Application Connection is checked and click OK.
-
Configuration service application association default MMS Go back to the navigation settings (~/_layouts/15/AreaNavigationSettings.aspx) and Create Term Set; it will create the System Navigation successfully.
Successfully created term set