REMnux is a lightweight Linux distribution for assisting malware analysts in reverse-engineering malicious software. The distro doesn't aim to include every useful security-related tool. However, the analyst may wish to add additional utilities to REMnux based on his or her needs.
The easiest way to use Metasploit on REMnux is to run Metasploit Framework as a Docker container. To accomplish this, follow the instructions in the following article: Run Metasploit Framework as a Docker Container Without Installation Pains.