upload/admin/model/user/user_group.php Line 86:

function removePermissions( string $route ): void {

shoud be

public function removePermissions( string $route ): void {

You are right, thanks for spotting it. Now fixed.

@mhcwebdesign, are you sure we need to remove permissions when installing the extension? Because now $this->model_user_user_group->removePermissions() called from install(), so it will remove permissions added by the extension itself. Because, for example, in my extensions, I add a permissions in the extensions, so that after installation I don't have to set them manually...
So, i'm sure we have to call $this->model_user_user_group->removePermissions() from uninstall().