Amazon discontinued .mobi support and the ability to use send-to-Kindle for .mobi files in 2022. Kindlepreneur has a great article detailing how you can send your ePub to your Kindle device which may work for you: https://kindlepreneur.com/mobi-is-dead/
If for any reason you still need a .mobi version of your file, we've had great success with converting Atticus exported ePub files to mobi using the free Kindle Previewer. We've even created a little tutorial on getting it to work, which you might find helpful: https://www.atticus.io/convert-epub-to-mobi/