Try removing libswresample-free and libavutil-free, and then try installing ffmpeg again.
Alternatively, you can do as it suggests with the --allowerasing parameter, so:
dnf install ffmpeg ffmpeg-devel --allowerasing
obviously review the output after this to see what it is actually going to do (in case it decides to remove a whole host of other packages as well).
Personally I prefer to manually remove the conflicts than use the allowerasing parameter, but that’s just me. In some cases, it could be a lot of hard work doing it manually, and so the parameter comes in handy, but caution is required when using it.