When Itry to send an email from my gmail account, a message pops up asking for my username and password as Mail wants to use the "system" keychain

Hello mary-anne67,

If your email account is not working, try removing it from the Mail application and adding it back. This can often resolve unexpected behavior with email accounts.

1. In the Mail app on your Mac, choose Mail > Settings, then click Accounts. 

2. Select an account, then click the Remove button .

3. Note: If the account is used by other apps on your Mac, you’re asked to remove the account in Internet Accounts settings. Click the button to open Internet Accounts, then turn Mail off for the account. To stop using the account in all apps, make sure the account is selected, then click the Remove button .

Log out of or remove email accounts in Mail on Mac

In Mail, choose Mail > Add Account, select an account type, then enter your account information. Make sure the Mail checkbox is selected for the account.

Add email accounts in Mail on Mac

If the password is still not working, check directly on your mail providers website to make sure the password is working there. This will help us to ensure this is not a login issue.

Cheers!