Bit instructions for the HC(S)08 use a fixed number between 0 and 7 for the bit. This is an immediate value, not a variable that contains the number. In your case, RXBITCNT should be EQUed to the specific bit number. To do what you want, you'll need to write a small routine that will move a bit to correct position (based on RXBITCNT) within the byte, and using LDA / OR (for BSET) or AND (with inverted mask for BCLR) / STA instructions, update the destination variable (RXBUF+RXBYTECNT) which again must specify an address directly (not as pointer, as I suspect you're trying to do by adding RXBUF + RXBYTECNT).
Also, some assemblers expect expressions to be inside brackets or similar decorations.