Which version of Exchange are you using and which CU?
It looks like an old issue that occur with Exchange 2013, please upgrade to latest CU and see if the issue resolved.
Some other options:
What changes did you make to your server? If you install some third-party tools, uninstall them and have a reboot.
Are you using the administrator account?
Try recreating powershell virtual directory as this blog suggests: https://www.alitajran.com/recreate-virtual-directories-in-exchange-server/#Recreate_PowerShellVirtualDirectory
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