Unable to change Android build number, only version name. Search console won't let me submit a new build.

When we go to submit a new internal testing build, and upload the .AAB file, this is what is returned by the search console.

We can't change the version number, only the version name, e.g
123 (version name here)

We need to be able to edit the 123, because it's not incrementing correctly.

How can I change the build number, there's no option within the Build/Download section to do so?