To help keep your account secure, Google will no longer support the use of third-party apps or devices which ask you to sign in to your Google Account using only your username and password. Instead, you’ll need to sign in using Sign in with Google or other more secure technologies, like OAuth 2.0. Learn more
What do you need to do?
Email software, like Outlook 2016 or earlier, has less secure access to your Gmail. Switch to Office 365, Outlook 2019 or newer, or any other email software where you can sign in using Sign in with Google.