With the MCS loader & a 4MHz oscillator, make sure you're programming the 877 with the 877 MCS loader firmware for 4MHz.

Add these to the top section of your code;

DEFINE LOADER_USED 1
DEFINE OSC 4

Setup the MCS loader as the programmer. Then click the Compile/Program button from within MCS.

If you have not wired the auto reset circuit as shown in the MCS loader help schematic, then you need to cycle power when this message pops up, or have a switch to /MCLR to reset the PIC during the boot loading process.

Assuming you have wired the MAX232 correctly, have a good serial cable/connection, etc, it should work fine.