Hello Sedef Fortacı Behar,
The error message "We couldn't sign in to Google IMAP" in Outlook 2016 is caused by Google’s deprecation of "Less Secure Apps" (Basic Authentication) combined with an older build of Outlook 2016 that fails to trigger the modern OAuth2 browser sign-in window. Essentially, Outlook is sending your username and password directly to Google, and Google is blocking the connection because it expects a modern token-based exchange, not a raw password.
To resolve this immediately without complex software updates, you must switch to using a Google App Password. This requires you to log into your Google Account via a web browser and navigate to the Security section. You must verify that 2-Step Verification is enabled; if it is not, turn it on, as the App Password option is hidden until this security feature is active. Once 2-Step Verification is running, search for "App passwords" in the Google Account search bar or locate it within the 2-Step Verification menu. Select "Mail" as the app and "Windows Computer" as the device, then generate the password. Google will provide you with a unique 16-character code. Return to Outlook 2016, and when prompted for your password during the setup, input this 16-character code instead of your regular Gmail password. This bypasses the Modern Authentication requirement and authenticates via the IMAP protocol directly.
If you are determined to use your standard Google password, your copy of Outlook 2016 must be updated to support Modern Authentication (OAuth2). You need to verify that your installation is at least Version 1704 (Build 8067.2115) or higher. Many volume-license MSI versions of Office 2016 do not support this feature natively without registry modifications to the HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity key to enable ADAL (Active Directory Authentication Library). If the App Password solution does not work, please reply with your specific build number found under File > Office Account > About Outlook so we can look at forcing the OAuth2 prompt via the registry.
I hope you've found something useful here. If it helps you get more insight into the issue, it's appreciated to accept the answer. Should you have more questions, feel free to leave a message. Have a nice day!
VP