Sharing across a network - AFP doesn't function

iMac with Catalina 10.15.4 with USB external HDD

MacBook with El Capitain 10.11.6

I have a small home network and want to share files from the iMac (Catalina) USB external HDD to the MacBook (El Capitan). This used to work but I haven't done it in a long time.

Just been trying to get it working again, in particular, I want to share files from the USB external HDD. Using AFP, it won't work - the MacBook OS tells me I don't have the necessary permissions.

However, if I change to SMB sharing, it all works just fine.

Apple seems to regard SMB as a less secure protocol than AFP but seems to be forcing me to use SMB to share these folders.

Is there any way I can share via AFP?

Thanks