Customize Term Set error: The default termstore for this site cannot be identified

When creating a site column of type Managed Metadata, and selecting "Customize your term set" under the Term Set Settings section, an alert box may pop up with the error:

This is because there is not a Managed Metadata Service Connection set to be the default storage location for column specific term sets. To set one as the default storage location, go to Central Admin, Manage Service Applications, and find the Managed Metadata Service desired, select the "Connection" below it and click Properties in the ribbon:

On the Properties modal window, check the checkbox "This service application is the default storage location for column specific term sets:

Then a site column using custom term set within a site can be created.