This issue can occur when using when you are using a SQL Change Automation Azure DevOps add-on, using an older task version, and the self-hosted agent you are using cannot download the SQL Change Automation Powershell module from the Powershell Gallery.

This was caused by a recent change in the Powershell Gallery TLS Support:  https://devblogs.microsoft.com/powershell/powershell-gallery-tls-support/

Change task version number to 4.x (which connects using TLS 1.2)