Basic Package header

[timeb]

I have been trying to take the last four bytes of the mac address as source and destination address to send data from one Lopy to a second LoPy. I created a package header as the following:

_LORA_PKG_FORMAT = "LL%ds"
_LORA_ACK_FORMAT = "LLB"
SRC_MAC_ADDR = ubinascii.hexlify(lora.mac().strip()[-4:])

Destination address

DEST_MAC_ADDR = b'9cc15603'

** pkg = struct.pack(_LORA_PKG_FORMAT, DEST_MAC_ADDR, SRC_MAC_ADDR, msg)

But when I try running it complains saying ValueError: unsupported format for the line with **. Is the method I followed wrong?