I guess you mean you can't create an Ubuntu or Kubuntu community within lemmy.kde.social, the instance where you created your account?
I have no idea about lemmy.kde.social but most instances don't allow any user to create their own community, in that case I suggest you to send a DM to one of the mods of this instance, or wait for them to contact you from this post, or in the worst case, create an account in an instance that does allow any user to create communities and create it there.
For now I think it might not be necessary, because there are multiple Ubuntu communities in different Lemmy instances and there is a Kubuntu community in this lemmy.ml..