Build-essential conflicts

When trying to do apt install build-essential I run into an error. I don’t want to break packages so I’m asking here does anyone know how I can resolve this to install gcc?

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 cpp : Conflicts: gcc (< 4:13.2.0-3) but 4:10.2.1-1 is to be installed
 g++ : Depends: cpp (= 4:10.2.1-1) but 4:13.2.0-7 is to be installed
 gcc : Depends: cpp (= 4:10.2.1-1) but 4:13.2.0-7 is to be installed
E: Error, pkgProblemResolver::Resolve generated breaks, this may be caused by held packages.

To resolve this, it would be useful to know more about your system. Can you show the output of this:

apt-cache policy cpp build-essential

Are you running vanilla sysimage-v4? What is inside your /etc/apt/sources.list and /etc/apt/sources.list.d?

The versions of cpp are even older than what is in current Debian stable so maybe your system is not up-to-date or mixing different repos?

sudo apt-cache policy cpp build-essential
  Installed: 4:13.2.0-7
  Candidate: 4:13.2.0-7
  Version table:
 *** 4:13.2.0-7 100
        100 /var/lib/dpkg/status
     4:10.2.1-1 500
        500 bullseye/main arm64 Packages
  Installed: (none)
  Candidate: 12.9
  Version table:
     12.9 500
        500 bullseye/main arm64 Packages

my /etc/apt/sources.list is

deb [arch=arm64] bullseye main contrib non-free

and /etc/apt/sources.list.d/mntre.sources

Types: deb
Suites: reform
Components: main
Architectures: arm64
Trusted: yes

I am running
Linux reform 6.6.15-reform2-arm64 #1 SMP Debian 6.6.15-2+reform20240215T125439Z1 (2024-02-15) aarch64 GNU/Linux

solved. Changed bullseye to trixie and everything showed up.

Changing your apt sources to Trixie will make more things work but other things will still break. You have configured as your source for the MNT related packages but those are built against unstable so there will still be situations (mostly when it involves libc6-dev, linux-libc-dev and friends) where you will run into installability problems.

You can keep using testing instead of unstable and it will probably work most of the time (definitely better than bullseye) but just keep this in mind in case you run into more problems in the future.