Ah, I reformatted your post to use the ‘code’ tag for the log messages and I do see an error there.
(to format as code, highlight your text and select the 'code' button</>above the editor window)
In locating your transform, the first forwardslash makes this a qualified path rather than a relative path.
TRANSFORMS="\__Download\SolarWinds.mst"
That would look for the __Download folder in the root directory of the hard drive. You need it to be relative to the working directory, so remove the forwardslash in front of __Download
Usually we could use relative paths to specify both the MSI and the MST transform. In your case for the MSI, you’re using a fully-qualified path but it’s working because you used a relevance substitution to give the full path to the MSI
"{(pathname of client folder of current site) & "\__Download\SolarWinds-Agent.msi"}"
You should have been able to specify the whole command as
wait msiexec.exe /i "__Download\SolarWinds-Agent.msi" TRANSFORMS="__Download\SolarWinds-Agent_AkerBP_A21-P1-MON004.mst" /qn
