Hiya, Sweet spot work is defined as effort taking place at 84-95% of your FTP (according to Trainer Road, I’ve also seen it described as work in the 83-97% range on training peaks) but the main point is that you have a wide window of effort there to work within. This is important as your FTP will increase as you progress consistently through the plan
By it’s nature, sweet spot work should be difficult but achievable, and repeatable, and is at a level where you can recover quickly, so you can do them on a daily basis and still see recovery and adaptation.
If you are worried that your FTP is too low then I would suggest reviewing after a couple more workouts, and then if you feel it is too easy then bump your FTP manually by 5 watts, or you could retake the ftp test 
also take a look at the topic below where your question has been discussed