I have No Idea. I get the same error as you when I try to do it.
However I found that I can copy and paste it as many times as I like. Then change whatever I want within the event, even add more OR statements.
What I do is, I open the project with the 'OR' statement in the desktop version of C3. Then I open the current project I'm working on in my web browser. (You can have more than one C3 project open at one time) Then I copy the entire 'OR' event from the project I have open on the desktop version of C3 and paste it into the project I have open in the Browser version of C3.
I don't know why it works, but it does.
Here's the statement for you. You can copy and paste it into any project and it works.