WordPress back-end says “sorry, you are not allowed access to this page”

Hello Hayley-

The most common cause of that error message is an invalid nonce code. Without getting too technical, a “nonce” is a “number used once” that WordPress uses to keep unauthenticated users from accessing your back-end. Most of the time, this error can be resolved by clearing your browser cookies and cache.

Occasionally, this error is caused by a malfunctioning plugin. If this issue just started recently, try disabling and plugins you’ve recently added or updated. Here are two ways you can disable plugins without access to your dashboard:
Disable plugins with FTP or File Manager
Disable plugins with WP-CLI (SSH)