Pretty sure you have AUR packages, right? Then go up the chain and remove the AUR package requiring ffmpeg 3.4 (which itself is not part of official Manjaro packages).
Update the system.
Then reinstall your AUR package.
Pretty sure you have AUR packages, right? Then go up the chain and remove the AUR package requiring ffmpeg 3.4 (which itself is not part of official Manjaro packages).
Update the system.
Then reinstall your AUR package.