Seven Segment Flickering

Projects General Guidance
4 
        //  digitalWrite(STCP_pin, LOW); delete this line
        shiftOut(DS_pin, SHCP_pin, MSBFIRST, dec_digits[(buf[i] - 0x30)] + 0x80);
        //  digitalWrite(STCP_pin, HIGH); delete this line
            
       }
       else
       {
        //  digitalWrite(STCP_pin, LOW); delete this line
        shiftOut(DS_pin, SHCP_pin, MSBFIRST, dec_digits[(buf[i] - 0x30)]);
        //  digitalWrite(STCP_pin, HIGH); delete this line

Delete the lines indicated above.