Variables & Data: The Scoreboard Mistake Almost Everyone Makes First
CREATE and CHANGE look interchangeable until the interpreter stops you. Here's why they aren't.
The Variables & Data set's first real trap isn't arithmetic -- it's reaching for CREATE a second time on a variable you already made, the way you might reassign a value in plain English ("make score 10 now instead"). Pseudocode treats CREATE and CHANGE as two different jobs: CREATE introduces a name for the first time, CHANGE gives an existing name a new value. Try to CREATE the same name twice and the interpreter stops you before it stops making sense to a person reading it.
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Run that and the second line fails with a message naming the fix directly: give it a new value with CHANGE score TO ... instead. Swap that second line for CHANGE score TO 10 and it runs clean. The reverse mistake exists too -- CHANGE-ing a name you never CREATE-d -- and gets the opposite-shaped message: create it first. Between the two, the rule becomes easy to keep straight: CREATE happens exactly once per name, CHANGE happens as many times as the value actually needs to update.
Everything above runs in this page, on your own machine. There is more of it on the practice pages.