You may be unable to log in to the Zoho CRM Plugin for MS Outlook if you have entered an invalid email address and password.
In such a situation:
1. Check if you are using the correct login credentials.
2. If you have enabled two-factor authentication (TFA), you will need to enter the unique verification code generated by the Google Authenticator App on your mobile.
Read here to know more.
3. If the credentials are correct and you're still unable to login, check if you're using Google Apps.
3. If you're using Google Apps, check if you've created a username for your Zoho account.
4. Go to
https://mail.zoho.com, create a new username for the account and try the same in Outlook.
5. If the issue persists,
zip the folder " C:\Users\<pc user login>\AppData\Roaming\zohocrm_outlook" and send it to support@zohocrm.com.