SharePoint/ Teams Private channels - can no longer manage or create groups

This was a change implemented by Microsoft:

"(Updated) SharePoint channel sites - Temporary permissions editing
MC282480 · Published Sep 1, 2021 · Last updated Sep 23, 2021
Service
SharePoint Online

Message Summary
Updated September 23, 2021: In two weeks, the window for editing channel site permissions from SharePoint will close. Once this window closes on October 7th 2021, all channel site level permissions will need to be managed from within Microsoft Teams.

We recently provided guidance (June - MC261534) “SharePoint: Updates for Microsoft Teams connected team sites” about how to edit permissions for SharePoint team sites that are connected to a channel in Microsoft Teams.

In the post, we specified that the addition, removal, or the editing of permissions for a channel site would need to be done in Settings for the team in Teams.

To allow team owners more time to adjust to this change, we have temporarily enabled (for the first three weeks of September) users to continue to edit site permissions in the SharePoint team site. This will allow you to audit and remove permissions that have been added directly to the SharePoint site.
Key points:

This temporary update is available to all customers until mid-to-late September 2021.

We will alert you via a follow-up Message center post to announce when this window is closing.
How this will affect your organization:

Starting this week, team owners can edit site level permissions for the channel site for the next three weeks. After three weeks, team owners will need to manage channel site permissions in Settings for the team in Microsoft Teams.

What you need to do to prepare:

You do not need to do anything to prepare. This is a temporary change to the way site permissions will be managed for Teams connected experiences. In a few weeks, all channel site permissions will be managed in Microsoft Teams Settings."

Learn more:

Manage Teams connected sites: https://docs.microsoft.com/SharePoint/teams-connected-sites
Private channels in Microsoft Teams: https://docs.microsoft.com/en-us/MicrosoftTeams/private-channels
"A private channel site syncs data classification and inherits guest access permissions from the site of the parent team. Membership to the site owner and member groups are kept in sync with the membership of the private channel within Teams. Site permissions for a private channel site can't be managed independently through SharePoint."