After a fresh installation of uCommerce you may notice the default "store" instance; uCommerce.dk. Like me, you may think to yourself "Hey, I'll delete the default store and create mine," whatever that store should be. But in uCommerce there is a bug where if you delete that, or all stores, such that you have no stores configured you cannot then create one. Somewhere in their code, I haven't decompiled it, they get a "Sequence contains no elements" exception.
The work around for now is to create the new store first, then remove the undesired one such that you always have one or more stores existing.