Install Transmission with web interface on Manjaro Gnome 20.0.3

Transmission by default doesn’t have the web client enabled in it. However, the packages transmission-gtk and transmission-cli  are available via system repositories and  would work for you with a bit a more efforts then on Debian systems. What exactly I mean is  to create manually service unit for transmission-daemon according to systemd requirements and tune several kernel parameters via sysctl to avoid errors at start up of transmission-daemon service.

After complete installation of Manjaro add one more package  
$ sudo pacman -S transmission-cli  

Make sure that your system is configured as follows
~ >>>  pacman -Qs transmission-cli                              

 local/transmission-cli 3.00-1
 Fast, easy, and free BitTorrent client (CLI tools, daemon and web client)

~ >>>  pacman -Qs transmission-gtk                          

  local/transmission-gtk 3.00-1
 Fast, easy, and free BitTorrent client (GTK+ GUI)

~ >>> which transmission-daemon                              

  /usr/bin/transmission-daemon

~ >>> ls -l  /usr/bin/transmission-daemon                          -rwxr-xr-x 1 root root 608920 мая 23 11:43 /usr/bin/transmission-daemon

Now create service unit for transmission-daemon per systemd requirements :-

Change directory to /usr/lib/systemd/system

[boris-pc system]# pwd

/usr/lib/systemd/system

and manually create file  

transmission-daemon.service

[boris-pc system]# cat transmission-daemon.service

[Unit]

Description=Transmission BitTorrent Daemon

After=network.target

[Service]

User=transmission

Type=notify

ExecStart=/usr/bin/transmission-daemon -f --log-error

ExecReload=/bin/kill -s HUP $MAINPID

NoNewPrivileges=true

[Install]

WantedBy=multi-user.target

Then reboot the box

  After system will come back on-line you are supposed to discover symlink under /etc/systemd/system/multi-user.target.wants

[boris-pc multi-user.target.wants]# ls -l transmission-daemon*

lrwxrwxrwx 1 root root 51 июл 11 18:31 transmission-daemon.service -> /usr/lib/systemd/system/transmission-daemon.service

and file 

/var/lib/transmission/.config/transmission-daemon/settings.json

The last one is read by created service and might be updated as follows

Per 

https://wiki.archlinux.org/index.php/transmission 

update /etc/sysctl.d/99-sysctl.conf and restart system without "Failed to set send/receive buffer"  errors  in status report of transmission-daemon service .

After updating settings.json file reload service transmission-daemon, restarting is also harmless. At this point you are all set