@river Now it works! Thank you very much for letting me know the root cause of this error. I wouldn't have managed it without your advice. Honestly I didn't expect to get an advice this soon!

@eu-Y Could someone tell me why this code wouldn't work?

{
    "dictName": "Weblio国語辞典",
    "windowUrl": "https://www.weblio.jp/content/<word>",
    "css": "header, footer, .adsbygoogle, .ad, .banner, #main-header, #main-footer { display: none !important; } body { margin-top: 50px !important; }"
}