This is very disturbing and currently breaking news in Germany. Have not seen it reported anywhere else so here's the article:
https://www.focus.de/politik/ausland/erkenntnisse-des-bka-focus-online-exklusiv-islamisten-planen-anschlag-auf-prinz-harry_id_257085859.html
It's an exclusive investigation by the reputable German media portal Focus. You many need to auto-translate the articles for details. It basically states that Al Qaida posted messages last week that explicitly called for attacks on the West, particularly the USA and naming Prince Harry as a target. The motive is presumably perceived revenge for the number of Afghani kills he disclosed in his book. The case is being closely followed and analysed by the German criminal police (BKA).
The rest of the article is more about Hamas and the overall growing climate of unrest between terror groups and state governments. It also mentions the growing concern for Lone Wolf operators who are not usually affiliated with a large, planned attack but decide to work on their own.