Troubleshoot Access Denied messages
If someone got an "Access Denied" message to a shared folder, the site collection administrator might have enabled "Limited-access user permission lockdown mode." To turn this off:
Browse to the site, click the Settings icon, and then click Site Settings.
Under Site Collection Administration, click Site collection features.
Next to Limited-access user permission lockdown mode, click Deactivate.
An Access Denied message can also occur for shared folders if the site is a publishing site. For info, see Access Denied when accessing a shared folder.
If a someone got an "Access Denied" message when trying to view access requests, the user needs to be added as either a site collection administrator or a member of the Owners group for the site. For more info, see Access Denied to Access Requests list.
If a user got an "Access Denied" message after they were removed from Active Directory on-premises and then added back, see Access Denied when a user account is synced to Office 365.