Same thing happened to me. I deleted notes to get down to like 45. Emptied the trash. Closed the program. Signed out and back into my phone and website. Nothing worked and I still can't add another note. If it's a glitch, that's fine. There have been way too many glitches lately.
I saw the writing on the wall. Evernote Free was increasingly becoming difficult to use. Always pushing you to upgrade to the pay version. I am done fighting with it.
I switched over to Google Keep. We shall see how long this one stays free.