Hi Readers, this article may help you to troubleshoot one strange issue that happens in SharePoint sometimes.

When we set up a new site collection in SharePoint online, by default it will create some permission group in the SharePoint site. Each group will have different permission level and some tasks can do by site owners only
Like that site owners only can approve the access request invitation from the site setting but sometimes even site owners can get the access denied message.
Initially, we need to verify the default members group has been associated with the SharePoint page properly by using the Access Request Setting (site Setting -> site Permission -> top ribbon access request settings)
SharePoint

Here you can see something is identified in red, which means SharePoint default group is not associated with the SharePoint page.

You can get the default permission group at https://tenant.sharepoint.com/_layouts/15/groups.aspx

But for restore default permission group, you need to modify the URL like https://Tenant.sharepoint.com/_layouts/15/permsetup.aspx

It will take you to the permission setup page

SharePoint

If you can see the Default group at following URL https://tenant.sharepoint.com/_layouts/15/groups.aspx, then you can select the Use an existing group option to restore the default permission group, or else you can create the new permission group and it will associate with your sharepoint page.

Once it associates, that red error will disappear from the Access Request setting

SharePoint

After that site owner can access the “Access Request page” to approve the users request.

But, sometime after following this step, site owners may not able to access it. On that time, ask site owner to send a request from the access form like below

SharePoint
And some one should log in as a site collection administrator to that SharePoint site and approve that access request and add it to site owners group, after that every site owner can view the “Access Request page”.