Explain the rule to me

i do not understand, why one solution works and other doesn't, i solved it on accident and i don't get it

https://preview.redd.it/c90bxrz5onmg1.png?width=523&format=png&auto=webp&s=f869f0056ff1e7c3981eff3cfd18880cee067de3

https://preview.redd.it/3h2875b1onmg1.png?width=564&format=png&auto=webp&s=0e214a7759db207debf9a9f8982d33d94985c4f1