Please make sure that you have the appropriate license to access Power Automate Desktop - Per user plan with attended RPA (https://flow.microsoft.com/en-us/pricing/)
Then, you can try deleting this file %localappdata%\Microsoft\Power Automate Desktop (preview)\msalcache.bin3 that stores your sign in info, and sign in again.
Also, you could check at the Windows Event Viewer if there is any info related to this error.
I hope the above helps!
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Power Platform release plan for the 2021 release wave 2 describes all new features releasing from October 2021 through March 2022.