I'm being asked to enter a valid email address on my Android mobile app and I can't sign in.

Hey thetompi, thanks for posting this here on our forum. 

Can you please reinstall the Dropbox app, if you haven’t tried this already? After that, please double check that both your email address and password don't contain any sort of typos or other characters.

Also, what’s your current plan? Is it possible you’re on the free Basic plan and you already have 3 devices linked? 

Let me know how it goes.