As the title says, I bought a license and entered the code in Help > Enter License.
Whenever I launch ST there is still the (UNREGISTERED) note, as well as the popup-nag.
Ensure you are entering all the lines, including BEGIN LICENSE and END LICENSE. It's important that no line breaks have been added nor removed: there should be 13 lines in total. If it's still not being accepted, please email firstname.lastname@example.org