We had same issue recently. Turns out for us it was because we had recently renewed our on-prem orchestrator server SSL certificate. It sounds like 2020.10+ activities in studio that pertain to retrieving info from orchestrator (Like get assets in our case), require you to update configuration on your identity server.
Long story short,we had to update the appsettings.Production.json file where the identity server was installed and put the thumbprint of the new SSL certificate.
For more background see: https://uipath-survey.secure.force.com/CaseView/articles/Knowledge/You-are-not-authenticated-error-with-Get-Asset-activity-with-UiPathSystemActivities-202010
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