Well, that's OK because the action of pedalling will tighten it up in use.
That conversion kit won't work for you, as it's for converting hubs that use cassettes on freehub bodies.
If you want a 'fixie', you should be able to thread on a single track cog in the size that you want.
If you want a single speed with a freewheel, then something like this would suit you:
You haven't told us what kind of bike this is for or how old it is, yet!
