Hello, last night my average Stresslevel was 8 under 15 the Sleepscore should be excellent but Garmin Connect shows "sufficient"!
Any ideas?

Could be just a translation problem. Switch the language to English on your phone, or look at it in Garmin Connect Web, where you can change the language easier. In English, at values below 15, I am getting "Good". I did not find any avg under 10 in the last days, so cannot confirm it shows then Excellent or not.
As I wrote, I am getting "Good" at values below 15. Not sure where the threshold for "Excellent" is, but it could be at 10. Looks like a wrong label is assigned to that level, but it plays no role, and has no impact on the metrics. You can see the true avg. stress value anyway, and the overall Sleep Score is unaffected either. Just a minor cosmetic glitch, but if you cannot stand it, report it to Garmin at https://support.garmin.com