Maybe you already know this way, if yes, just ignore my answer:
I had the same problem yesterday evening when it came to the “Please update the Sim via Microsoft store”-Message.
I just installed the XBox App called “XBox” in the store and signed in there. I waited for around 15 minutes and suddenly, the store updated the Sim itself within 20 seconds. After It I was good to go for the Marketplace update in the sim itself. Seems like The Update is “wired” to the XBox App and without it, the store doesn´t check for updates…