Hi there! The Tag source button makes it so you can add a new .ini file that will hold various tags but if you just want to add a tag to the current list you are looking at you can add it with that green “+” button in the top left corner of the Gameplay Tag Manager. Just pick which source you want to add to (Most likely DefaultGameplayTags.ini) and give the name there and then press Add New Tag.
I hope this helps 