I was testing out some stuff and ran across this error when trying to add an SSL cert to an application:
"Cannot enable SNI SSL for a hostname foo.bar.com because current site mode does not allow it."
This unhelpful error is just because you have selected a Free version of the Application Service plan. It has nothing to do with the web app you're in right now, other than the meager fact that you want to add SSL to it.
Upgrade the plan to a Standard or Premium plan to get past this error.