I'm unable to do a patch scan on a newly installed client machine ?
list stays empty ?
Check the patch policy group machine(s) in question. And did you approve some patches under that policy group, if not then you need to.
What Kaseya version? 6.3 or 6.5? If 6.3, have you updated to the new patch management module?
Is the patch scan procedure failing (Check the procedure log)?
Have you specified agent credentials? do they work when tested?
If the patch source location valid?
Is the LANCache configuration correct?
Is the agent a member of a patch policy?
Does windows update work on the machine? (You may need to run windows update manually once, so that it updates itself to the latest build of the WUAU components).
Is this a windows 8 machine? If so you may need to subscribe to Microsoft Update first.
You might want to review community.kaseya.com/.../740.troubleshooting-failed-patch-installs-and-failed-patch-scans-and-incorrect-data-on-patch-status-page.aspx for some tips on troubleshooting patch scan issues. This won't cover all of the points Craig raised - all of which are things to check - but will at least assist with some of the common things to check.