Hi,
I am sorry to hear about your issue.
Do you get the error immediately after creating the staging site or did you make any changes to it?
Please enable the WordPress debug.log file on the staging site, try to open again the site and send me the debug.log file to our support system at:
https://wp-staging.com/support/
It should contain the exact error which is preventing your site from being accessed.
You can see what plugin or theme creates a fatal error 500 by activating the debug mode and logging. This will write the exact error and plugin name in a file called debug.log on your website.
How to activate the debug log and where to find it:
https://wp-staging.com/docs/enable-wordpress-debug-log-mode/