Is this mean that Portc.7 can not be used as my rx?
No. I was just pointing out why your original code didn't work as you expected.

It doesn't matter which pin you use for receiving serial data. You just need to understand why your original code isn't working as expected.

Every 3.2mS or so your program jumps to the SERIN interrupt routine. And it just sits there until it receives a byte of data.

This means it will take a very long time before your first loop can finish because it gets interrupted too often for your first loop to complete.