If you can not log in to the staging site created by WP Staging check out first if you have any security plugin running on your staging website like WordFence, WP Spamshield or anything similar.
Create a new staging site and disable these plugins from the staging site by excluding them from the WP Staging file selection manager:
Alternatively, you could try to disable the WP staging authentication from WP Staging > settings.
If this still does not resolve your issue we can offer you to have a look at your website to find out why you can not log in.
In that case please open a support ticket and send us the system info log from wp staging > tools > system info
Also, let us know if you get any specific error message when you try to log in.