You are not allowed to access this page.

Some good points to address this issue is listed in the article linked below:

https://premium.wpmudev.org/blog/fix-not-allowed-access-page-error-wordpress/

You can start with enabling WP_DEBUG to true in wp-config.php and also check error logs for anything unusual.

An extensive troubleshooting guide – https://perishablepress.com/how-to-troubleshoot-wordpress/