There are very few Bonvoy hotels in Scotland: I think it's a total of 15 properties, all concentrated around Edinburgh, Glasgow, and Aberdeen, and the 1 Trump Turnberry golf property in the Ayrshire area south of Glasgow.
If you want options in the Highlands aside from those, you largely will need to search for smaller boutique properties or non-major chain hotels. Tripadvisor and other search engines may be helpful for you depending on your budget. You also would do well to look into the Scotland Forum here on FlyerTalk for idea/suggestions.
(For perspective, Hilton has 25 properties in all of Scotland, and they're concentrated in many of the same areas as those from Marriott, with a couple more in Dundee and Stirling, too.)