if you configured vCloud Director to use a VXLAN based Network Pool, and then downgradd your License and VXLAN was removed ... yes, you will not be able to create an Isolated Networks because you don't have a license for it.
If this is your long term permanent license, then you need to make some changes (use a VCNI pool instead of VXLAN).
couple questions:
1. Are you using vCenter 5.1 (any version)
1b. if so, do you have a vCloud Suite license installed in vCenter 5.1?
1c. if so, can you apply that license to the vCloud Network and Security Asset and see what it says then?