Per my test, if I changed the group name, changed the group from public to private and removed all co-workers after creating the group, the same error occurred when I tried to delete the group.
As a workaround, you could go to Teams to delete the group. Here are steps:
1.Log in Teams with your Microsoft 365 account.
2.Teams -> Click the far-right hand edge of the Team name to display the options menu (...) -> Delete team.
3.The group and associated site will be automatically deleted.
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