Thanks for sharing the details, 유재희! IMAP could have been disabled in your Gmail settings. Please check that:
Log in to Gmail in your web browser.
Click the Settings then See all settings.
Go to the Forwarding and POP/IMAP tab.
Under IMAP access, select Enable IMAP.
Click Save Changes.
See if this helps. If you need further assistance just let me know.
Best regards,
Kimberly