Reset Password:

Ensure the client has reset the password successfully.

Double-check that the new password is being entered correctly.

Check Firewall and Security Software:

Verify that there are no firewall or security software settings blocking the MT5 application.

Temporarily disable antivirus or security software to check if it is causing the issue.

Update MT5:

Ensure that the client is using the latest version of the MetaTrader 5 platform. If not, recommend updating to the latest version.

Clear Cache and Cookies:

Clear the cache and cookies of the web browser being used for logging in.

Check Internet Connection:

Ensure the client's internet connection is stable and not experiencing issues.

Test on Another Device:

Suggest trying to log in from another device to see if the issue is specific to the Mac device.

Verify Mac System Requirements:

Confirm that the Mac device meets the system requirements for running MetaTrader 5.

Check for System Updates:

Verify that the Mac operating system is up to date with the latest updates.

Advise to use the MT5 Web Platform as an alternative through this link
https://fusionmarkets.com/Platforms/Metatrader-5