I have Horizon 7 setup in my test environment and as far as I can see my setup has everything needed to create an Instant Clone Pool but when I go to add a Desktop Pool the Instant Clone option is greyed out.
Which options did you select in the wizard so far? On each screen you can see the available features - based on your selections - on the right bottom of the screen.
Also remember that Instant Clones require Horizon Enterprise licensing.
you need to have the enterprise license to activate this feature
try use Floating User assignment.