The Bitrise virtual machines have a certain amount of free storage available, which is generally enough for the purposes of most builds. However, for apps with large repositories or that generate large files, the space might not be enough.
If you need more storage space, you can uninstall certain pre-installed tools that you don't need. To do so, add a Script Step at the start of the build that contains the uninstall commands.
You can remove, for example:
Android tools (When only using iOS on the VM).
Simulators not in use.
Check the System Report of any given stack to see the list of pre-installed tools.