Hi, and welcome to the forum DonLuis.

Proteus lets a lot of things work in the virtual world, that will not work in the real world. For the real world 16F877 you need an oscillator to be connected, or a resonator. Resonators already have the capacitors needed, but aren't quite as accurate as real crystals. If you get a 8 to 16 mhz crystal, you will need two 10 to 20 pf capacitors. See page 145 of the datasheet (147 in adobe).