Dear Popa,

Your code starts as

AJMP START
        
START:  MOV TMOD,#20h
        MOV SCON,#50h
        MOV TH1,#0F4h
        MOV TL1,#0F4h
        SETB TR1

        MOV COUNTER,#01h    ;init counter
        CLR RECEIVED
        CLR VALID_TAG
        SETB P2.0                ;the LED must be in off state

FIRST_BYTE:     JB TI,TRANS      ;if transmission is over jump to trans
                MOV A,SBUF
                CJNE A,#0Ah,LAST_BYTE   ;check first byte
                MOV COUNTER,#01h
                AJMP RECEV
 

Also near end you have something like

RECEV:  CLR RI
        RET

TRANS:  CLR TI
        RET
 

You are branching to TRANS / RECEV using "AJMP", then you should also come back using "AJMP" (or "LJMP" or "SJMP")

If you want to use "RET" then you should "ACALL" or "LCALL" RECEV/ TRANS.

Regards,
Mahesh