Why can't I set digital IO of PIC16F628
It's hard to tell what's going on without showing us your entire code, and a schematic of your physical implementation.

However this is what you can look into:


You should add a for-ever loop at the end of your code. Your microcontroller might be shutting off after one run. Add at the end a while(1) or for(;;)
Try adding the following code to your initialization (or whichever format your micro controller header defines these registers)


ANSEL  = 0; /* Configure analog pins as digital*/

CMCON  = 7;    /* Disable comparators*/


I haven't used microchip controllers under MikroC, but are you sure it's TRISA and PORTA instead of TRISO and GPIO?
Make sure you configured the internal oscillator (INTOSC) appropriately. 
You can configure it by settings bits 0-2 (FOSC2:FOSC0) of CONFIG register
to either101 or 100for INTOSC