For you or anyone else wondering about their default skin, they are not working right now because EA came out with an update on October 3rd. This update re-made the base skins. You must wait for the person who made your default skin to make an update. Making default skins for the Sims 4 is probably the most difficult custom content to make. They may need to start from scratch, so it may be weeks before they can get them working again.
I prefer Luumia Vanilla Skins. Updates have already been provided. First on the 12th, the 13th, and again on the 14th. However, they still are not working. Just keep checking back to whatever page you get your favorite skin from. They will be fixed eventually. Also, be certain to thank your creator. It's a lot of work.