It may be noted that dropbox or any variant thereof is not present in the official repositories.
To my knowledge it was never there.
There are some packages in the AUR though.
This would be roughly the approach.
Did you sort mirrors?
For whatever reason there have been a few threads about ‘enabling AUR in pamac does not make AUR available’ … but most or all eventually resolve with no clear rationale.
Like here
You may also consider alternative package managers and/or aur-helpers.
yay, paru, and trizen are among the favorites for aur-helpers that are frontends for pacman.
pacman being the default package manager of Arch, that is included in manjaro, that is exceedingly reliable but cli-only.