This is a tricky problem to solve if you don't have access to enough information, as there are many different factors that may be causing your Magento cloud CLI account login to fail.
Here are some strategies you can use to troubleshoot the issue:
Check the Logs: Check in the debug logs to see where the login-failing cycle is stopping. Magento has an article about log-related topics and how to access the same. Go through it and confirm the details https://devdocs.magento.com/guides/v2.4/config-guide/prod/prod_trouble_logging.html
Investigate the Network: Make sure your network is not blocking requests between the hosting server and your server. You can also refer to this article on how to troubleshoot network related issues. https://support.magento.com/hc/en-us/articles/206221727-Troubleshooting-network-issues-in-Magento
Login Credentials: Have you recently changed your username/password for the Magento Cloud CLI account? If yes, then that change might be causing the issue. Make sure you are entering the updated credentials during login.
Review the Connection from your Magento Admin: Check the connectivity from your Magento Admin by navigating to System > Web Setup Wizard. If the connection is unable to establish, it may be the cause of your login fails.
Get in Touch with Magento Hosting support: If you still can't figure out the cause, try getting in touch with your hosting provider's technical support. They may provide better support you require to solve the problem.
I hope these strategies help you in troubleshooting the Magento cloud CLI account login issue you are facing. If you need more information or help, do let us know.