Hi @Jessey Clarke ,
You would have to goto Credentials tile and create a credential in your Azure Automation account and then click Compile in your Configuration under State Configuration (DSC) tile and provide the credential name which you have provided while creating the credential in Credentials tile.
For more information with regards to this topic, refer below articles.
https://learn.microsoft.com/en-us/azure/automation/automation-dsc-compile#azure-powershell-1
https://learn.microsoft.com/en-us/azure/automation/automation-dsc-compile#work-with-assets-in-azure-automation-during-compilation
https://learn.microsoft.com/en-us/azure/automation/compose-configurationwithcompositeresources#compose-a-configuration
https://learn.microsoft.com/en-us/powershell/dsc/configurations/configdatacredentials?view=dsc-1.1
https://learn.microsoft.com/en-us/powershell/dsc/configurations/runasuser?view=dsc-1.1