What I did: Looked for a way to turn on spell check
What happened: Couldn’t find it
What I expected: Spell check to kick in automatically when I started to write a note
Things that might be helpful to know (Agenda version, OS and model, etc): iOS
On iOS, spelling should work the same as in all apps, I believe. It probably depends on settings you have in the Settings app. You should have the normal autocomplete to help with spelling too.
What about spellchecking in MacOS?
On macOS, first put the cursor in a note, and then go to Edit > Spelling and Grammar. Change the settings. After that, they should apply in all the notes.