Today I tried to do the FTP Test with my Edge 520 plus.
After the warmup, I stayed as good as I could within the upper and lower limit of each intervall.
When the 3rd intervall was over (225-245 watt) the garmin popped up a message saying "the FTP value could not be calculated. Please try again".
I tried a second time. This time the same message came after the second intervall.
What can be the reason why the garmin cannot calculate the FTP value?
Should I consider anything special during the intervalls?