During domain migration from Plesk server to PPA infrastructure you see the following warning:
Stderr is Database server '' not found.
This warning appears because the special provisioning attribute - 'WebHost' - was used. The 'WebHost' attribute is used to force Plesk Automation to provision web hosting and database services on the same node (the node with the mixed role - web and database server). In this case migration agent passes an empty value of Database server.
This is minor warning, you can ignore it.
You can find additional information regarding forcing PPA to Provision Web Hosting and Database Services to the Same Node in Parallels Plesk Automation Operations Guide.