The 'koha item type' is compulsory, but doesn't look it. It should be in red, and default to the top one on the list so that it's impossible to make an error that results in a mysterious message.
This occurs when you don't have an acquisitions framework set up that makes koha item type mandatory.
I am not sure changing the order is a good idea. The order is defined by the framework the same way at different places in Koha (cataloging, subscriptions with items in serials). There is also a bug report requesting a default ACQ framework. (bug 3841) This bug is duplicate or at least related closely to bug 4955. Marking some fields as mandatory in frameworks was a bit problematic in my last tests. Bug 3727 describes what I saw, but this was some time ago and needs to be retested perhaps.
If the there is no default ACQ framework then most users will not know that this is why they're seeing this problem. It's a bug in my book because there are no clues to the user as to why they have this error poppping up. And it is a duplicate, marking it as such. *** This bug has been marked as a duplicate of bug 4955 ***