can't solve 'endswith first arg must be bytes or a tuple of bytes, not str' error

Question

I'm having trouble concatenating bytes. I am attempting to communicate with an mass flow controller by the code below. But I keep getting the error: endswith first arg must be bytes or a tuple of bytes, not str

The code is:

class MKS946:
    def __init__(self, com_port):
        self.port = int(com_port)

    def __enter__(self):
        rm = pyvisa.ResourceManager()
        self.mks946 = rm.open_resource(f"ASRL{self.port}::INSTR")
        self.mks946.baud_rate = 9600
        sleep(0.1)
    
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.mks946.write("@253LOCK!OFF")
        self.mks946.close()


    def comm(self, command):
        """ Implement communication protocol """
        prestring = b'@253'
        #endstring = b';FF' 
   
        self.mks946.write(prestring + command.encode('ascii'))
        sleep(0.1)
    
        return_string =   self.mks946.read_bytes(self.mks946.bytes_in_buffer)
        return return_string
 
    def get_id(self):
        return self.comm('SN')

if __name__ == "__main__":
    with MKS946(4) as mfc_box:
        print(mfc_box.get_id())

The offending line is: self.mks946.write(prestring + command.encode('ascii'))

How can I concatenate the bytes in a simple way?