Error trying to Drag & Drop a link into a Dropdown Menu - Sharepoint 2016

Hi folks,

In Sharepoint 2016, I've created a new link on our top menu. When I click Edit Links, and then try to drag it into a dropdown in the same menu, and then Save, I get the following error. 

"There was a problem. Some of your changes may have been saved. The operation is not valid on the pinned term: 'Tools & Resources'"

"Tools & Resources" is the name of the dropdown I wanted to add the new link to. 

Any advice would be appreciated, thank you!