In your screen shot you should have Package Servers listed, what happens if you change "Assign package to:" to "Package Servers individually", are any Package Servers listed?
But I think you have made a mistake about Site and Package servers, you need to read the ITMS planning guide to understand their roles.
A Site Server can have a number of roles, one of which is that of Package Server, it holds copies of your packages so that clients can download the package from the local site server instead of the master copy which may be on another site. The design of the site server infrastructure should be done by someone who has read and understood the ITMS Planning Guide but very roughly you should have one Site Server with the Package Server role per physical site.
You should not make all your clients site servers or package servers, that is wrong.
If you only have 750 clients in total you only need one Site server with the Task Server role, this can be shared with a package server.