Sometimes Power BI requires sign-in to display reports within monday.com. The solution depends on which platform you are using.
Safari Users
If Power BI displays the sign-in screen and after signing in there is no update:

Follow the guidance in Some of my Safari users cannot see the content in an app.
monday.com Desktop App Users
Power BI must open the sign-in screen in a new window or browser tab. The desktop application does not support this, preventing report access.

Solution: View monday.com in a desktop browser instead.
monday.com Mobile App Users
The mobile app cannot open authentication in a separate window or tab, blocking report viewing.

Solution: View monday.com in a desktop browser instead.