I have the same problem in the German version Evernote 10.46.7
I have the personal plan, only 2 templates, and it worked before.
But today it didnt work anymore. I can see the "Add as a template" selection but it is grey and I cannot klick on it.
I tried severals things, like remove of the titel and make a new one. I tried on other notes to make a template out of it, but it doesnt change anything. It is not working.