My iCloud password keeps failing

Apple requires two-factor authentication and an app-specific password for third-party apps accessing iCloud (since June 2017). You cannot use your regular Apple ID password. Generate an app-specific password at appleid.apple.com → Sign-In and Security → App-Specific Passwords, label it 2Do, and use that in 2Do. If the password still fails (or you see a cryptic -1012), check:

• Your device's date and time is set to automatic. A wrong clock makes app-specific passwords silently fail.
• You're using your @icloud.com address, not @mac.com.
• You're using your primary Apple ID, not an alias.
• You haven't reset your Apple ID password recently — that revokes all existing app-specific passwords; generate a new one. If none of those help, remove the iCloud account from Settings > Sync in 2Do and re-add it with a freshly-generated app-specific password.