Asynchronous counter

The simulator doesn't know what value, '0' or '1', to give each of your flip flops at the start of simulation. So, it will give each the value of 'Z'. The result of any combinational logic when any of the input values is 'Z' will be 'X'.

In practice (on real hardware) this won't be an issue since all your flip flops will power up as either a '0' or '1' and everything will work.

So, I suggest you modify your code/schematic and introduce a 'reset' signal. When you assert this reset all the flip flops will take a known value ('0' or '1'). You may need to use a slightly different flip flop primitive. The one you've chosen doesn't appear to have a reset. However, there should be one that does.

Cheers,

Alex