You aren't allowed to use tags that haven't already been used
When you draft a new blog post, and pick the tag "foo", it prevents you from publishing, because "foo" is not a known tag.
It looks like you're only allowed to choose from tags that were used on the old blog?
The better behavior is to let people pick whatever tags they want to pick.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information