You can't sign in to this app because it doesn't comply with Google's OAuth 2.0 policy for keeping apps secure. You can let the app developer know that this app doesn't comply with one or more Google validation rules.

Previous discussion: https://discussions.apple.com/thread/254696428

Generally: upgrade macOS if the hardware supports it, use a different mail client, use the webmail interface, or encourage Google to lower or drop their requirements.

mikedard wrote:

Apple doesn't allow Mac Mail app updates, they only update the Mac Mail app with new operating system updates. When your platform is no longer supported the Mac Mail app doesn't update. Apple is at fault.

Pretty much no vendor can financially survive supporting old versions indefinitely. That makes new work and enhancement work somewhere between exceedingly expensive and clumsy to implement and to use, and impossible.

I find setting up my Gmail account as an IMAP account and using it in Mac OS Mail that way works fine on older operating systems. The presets for Gmail, AOL, Verizon, and Yahoo on older releases of Mail don't work as well as Other... selecting IMAP, as long as the server side of the service knows to give you your email in IMAP mode.

mikedard wrote:

How to comply with Google 2.0 policy, using Mail Version 11.5 (3445.9.7)?

A good start is by contacting google support.