In order to confirm, the user could navigate to Journey Builder check for an error which mentions - "We can't authenticate your request ... To Use Journey Builder, enable third-party cookies in your browser."
This is typically caused by either your Browser or an Extension blocking Third Party Cookies. To test this, try enabling third party cookies.
If allowing Third Party Cookies in your browser/extension does not resolve the issue, proceed to the following potential causes related to the browser/network being used:
1) Update the current browser to the newest version
2) Try using a different browser
3) Clear cache and cookies in your browser
4) Try using a different network
5) Use a different device
6) Work with your networking team to determine what may be blocking the connection
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