need help in desigining microstrip antenna.

Hello.
I need to design microstrip antenna with these dimensions 60.6mm x 55.5mm on FR4 and my requirements are gain and directivity should be higher than 6dBi and efficiencies should be higher than 90 %, the feed can be microstrip or coaxial.
i have used microstrip line feeding but i m not getting gain and directivity higher than 6 dBi,
so please somebody help me
i will be very thankful to you.
regards

This is physically not possible.

why not possible?
please kindly give me the reason.
thanks

The gain of path antennas is limited by physics.
In addition, using FR4 substrate reduces gain because of high dielectric losses in the substrate.

can you elaborate more about physics, actually i m gaining my knowledge in antenna so i need the logic behind everything.
because practical knowledge about the tools is not enough and i think i should have theoretical knowledge about the antenna.

Why don't you get a good antenna book first, before starting simulations?
This way, you can learn what you can expect from the different antenna types.