I don’t know what you mean by [B] command. If you mean *bug, you can read more about it here.
As for your question, you shouldn’t use “%” at all, only operators and numerical values.
*if (relationship_x >= 50)
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Edit:
I reccomend that, in addition to the wiki, you give the following resource a good look:
Master List: Links for Beginners
You can also check out the code of other games to help you better understand how choicescript works. Read about how to do that here and here.