Using fully defined sketches
There are a couple of ways you can fix this issue. First, you can always add all of the dimensions and relations to make this sketch fully defined. The problem with this is the next time you create a sketch, you will get the same error message. The best way to handle this issue is to change the setting. Go to Options, System Options, Sketch and uncheck “use fully defined sketches”
This is the preferred setting for SolidWorks. The reason for this setting is that you want to have the freedom to work with organic shapes. You will not use fully defined sketches when dealing with these complex shapes as it would make your modeling extremely difficult.