Welcome to the Dataiku Community.
I notice that in your example you are typing if else in mixed cases. Dataiku's formula language is cases sensitive. When I started using it that case sensitivity caught me off guard. Here is a prior thread on this topic.
@Manuel comments are great. DSS's does not provide an elseif syntax. The syntax is much more like MS Excel here is a thread on that topic there is a bit more of a discussion.
Please let us know how you are getting on with the formula language. Once you master it there are a bunch of powerful things that you can do.