If you need a desktop environment at OMV, I have good news and bad news for you. The bad news is that you CANNOT install a desktop environment on debian OS with OMV. The good news is that this has an easy fix, read on.
For all those who need a desktop environment and insist on installing a desktop on their OMV system. Sorry to say that OMV is incompatible with a desktop environment installed on the host for many reasons, it will end up crashing, this has been repeated over and over again in this forum. If you do, do not ask for help in this forum, you will not get help. No help is possible. This is not a whim of the OMV developers.
BUT, there is an easy solution. You just have to install this docker container.
https://hub.docker.com/r/linuxserver/webtop/
This container creates an image with a desktop environment that you can access from any browser. By default an image based on Alpine will be installed, very light but with certain limitations. If you need something more powerful use the ubuntu-xfce tag.
If you need to install any desktop application you can do it within this desktop. If you need to manage your OMV files, you just have to correctly configure the container volumes and you will have a graphical file manager.
This installation will not affect the operating system or OMV, you can use it without any problem, uninstall it, reinstall it ... Read the linuxserver documentation for its use, especially regarding security.
If you don't know how to install a docker container you can check here [How to] Prepare OMV to install docker applications