I have installed manjaro distro on my computer but I'm unable to install any .rpm packages and from my researches you can only install packages using pacman and AUR.What I'm asking is that is there any way to install rpm packages on manjaro considering it is derived from Arch linux?
Arch (and any based ditros on it) does not rely on
RPM  format, that's is in fact developed and used by Red Hat derivates.
There are some cases (e.g. this one ) in which a RPM package is used to install an application on Arch (and derivates) systems, but prior to do this, the RPM is extracted and handled/converted in the Arch flavour.