@Nicky182 If you'd like to offer a refund of any amount, just click https://www.airbnb.com/resolutions
This won't be the last time something like this comes up. Travelers who in pre-pandemic times might have usually jetted down to the Mediterranean for their holidays are now seeking out domestic destinations that they perceive as charmingly idyllic. They don't necessary consider that farm smells, insects, spiders, and mud are also part of the experience, and that life in rural communities doesn't revolve around leisure travel.
Your listing prominently features beautiful pictures of a sun-splashed garden and fragrant lavender blossoms, so it's not surprising that the guests thought they were in for lots of relaxing time outside enjoying the fresh country air, rather than spending the holiday inside their bedroom with the windows closed to keep out the foul odor. The guests aren't owed a refund just because they didn't like the smell of the air, for the same reason you're not responsible for bad weather or anything else beyond your control. But if the guests are displeased with your response, you'll have to accept that your perfect 5-star streak has come to an end. It's not the end of the world - nobody keeps a score like that forever.
You might consider working on the listing descriptions a bit to set more accurate expectations. There's nothing wrong with mentioning a few things that some people might perceive as negatives - if it puts guests like these off from booking, that can only be a plus.