1/ Echo-cancellation is usually not needed if using headphones
Some microphones have built-in echo cancellation. Adding echo-cancelling in PulseAudio would not be necessary and could be counter-productive
Some VOIP software has built-in echo cancellation, so adding another software algorithm could also be unnecessary and counter-productive
2/ Archwiki should have been more specific about adding commands to the end of /etc/pulse/default.pa
I suspect module-echo-cancel may need other modules loaded first
(module-filter-heuristics and module-filter-apply)
your screenshot does not show the end of /etc/pulse/default.pa
the final section has partial commands for default sink and source commented out
### Make some devices default
#set-default-sink output
#set-default-source input
The sinks and sources have to be loaded before setting one to be default
If you move the section you added to the end of the file the module should load correctly
use this command to get a list of loaded modules
pactl list short modules
[EDIT - final command was incorrect]