Hello!
After a few tries and some googling. I figured out a way to mount AFP share on my Raspberry Pi 2 running OSMC.
From what I understood here, the reason it says: “Could not pick a matching UAM” is that some dependency (libgcrypt?) are missing and therefore, some encrypted authentication methods do not seem to work.
Here is what I did to make it work. I’m not an expert so there might be unnecessary things getting installed. Since I couldn’t find an arm package of afpfs-ng, I had to install build-essential and compile it myself. If anyone has a link to a package, I would be quite happy 
First, make sure apt is up to date:
sudo apt-get update
Then install all the dependency:
sudo apt-get install libgcrypt-dev libgmp3-dev libfuse-dev libfuse-dev libreadline-dev ncurses-dev build-essential
Download the afpfs-ng latest source from here and put the tar file in your home folder. Then uncompress file and compile.
tar xvjf afpfs-ng-0.8.1.tar.bz2
cd ./afpfs-ng-0.8.1
./configure && make && sudo make install && sudo ldconfig
After this is done, you should be able to mount your Time Capsule with a command like this:
mount_afp afp://username:password@TCname.local/sharename /mountpoint
I hope this will help you!
I am now trying to auto-mount the afp share on boot but with no luck.
I added this line to /etc/fstab/
afpfs#afp://username:password@TCname.local/sharename /mountpoint fuse user=osmc,group=fuse 0 0
I also created the fuse group
sudo groupadd fuse
and “allowed other” in /etc/fuse.conf by uncommenting the line
user_allow_other
After that, the command
sudo mount -a
will mount the AFP share with the following output/error:
Unknown option exec, skipping
Unknown option suid, skipping
Unknown option dev, skipping
Unknown option dev, skipping
Unknown option suid, skipping
Mounting TC.local from TC on /home/osmc/TC
Mounting of volume TC of server TC succeeded.
Unfortunately, when I try to reboot, I get a “local file system dependencies failed” error and OSMC doesn’t boot.
Any help will be greatly appreciated!
Renton
EDIT: I figured it out.
As I expected, the auto mount was trying to mount while the network wasn’t available. Putting x-systemd.automount and noauto as options in the /etc/fstab line solved the issue.
afpfs#afp://username:password@TCname.local/sharename /mountpoint fuse user=osmc,group=fuse,x-systemd.automount,noauto 0 0