A lot of people have the same problem and there are many tickets on this topic but I just created a new post to highlight this big issue once again. I have Garmin Forerunner 235 and everything works fine with Garmin Connect. However, although I allowed all permissions for the Garmin Connect app and followed all methods people have tried, step counts and other data are not visible in Apple Health app. Many 3rd party apps that use step count and other health data can only read from Apple Health app.
Apple experts refer to Garmin team for this problem, while Garmin experts refer to Apple team. From one point of view, if I bought an Apple Watch for tracking, everything would be perfect and I could easily read my data from Apple health. I chose the Garmin watch because I believed Garmin brand was reliable and its devices were known as professional. Therefore, I do not agree to fix this problem with Apple team, Garmin should support me and all other customers, and solve this problem immediately with Apple!
