We’ve had a policy that in order to encourage charms to be worked on as a group and that there’s enough differences (e.g. install commands are completely different and expected software versions like python versions are different enough) that best practice is to have split charms for different operating systems. Common code can be encapsulated with shared layers between the charms as a way to reduce the maintenance burden. However, you can’t submit charms for both ubuntu/windows or ubuntu/centos or centos/windows.
The charmstore is for shared best practices and while it can work locally we have more strict rules in the charmstore.
That’s the logic of what you’re hitting. We haven’t had many charms in this category and if you’re ok with sharing the repo it’s at I’d be interested in looking at how the differences between OS’s are handled.