@Osmium_Dresden The core of the OS (/usr, etc) is mounted read-only and managed by ostree, so you cannot install arbitrary packages using dpkg or apt. If a package can be unzipped and run from your home directory without installing, that might work for you.
See https://community.endlessos.com/t/command-for-compiling-c-program-code-in-terminal/1359 and https://community.endlessos.com/t/announcing-flapjack-developer-tools-for-endless-os/5442 if you are interested in using developer tools in Endless.
The other option, if you absolutely have to install a package is to run sudo ostree admin unlock --hotfix (and wait several minutes for the operation to complete). This overlays the ostree with a writeable filesystem to which you can install or modify packages. (With --hotfix, the changes you make are retained after rebooting.) The overlay is removed when the OS is updated, and any changes you have made are removed. This feature is only intended for advanced users who really know what they are doing, and we may not be able to support you if anything goes wrong.
Roddy