Hi! Your watch might have messed up some data, resulting in a corrupt .fit file. Try these steps:
1. manually upload https://support.garmin.com/en-US/?faq=Ht3ZP52Kju075uKvqTqu99 - connect your watch to your…
I am having the same problem right now with my newly purchased Forerunner 45. I even tried to do another activity and this new one appeared on my app. How to resolve this problem? Thank you
This worked for me, thanks so much! The file was corrupt and I used that url you posted and it fixed the file and uploaded it. Very happy, that activity was an important one for me to remember 
hi,
funny 
you are aware you asked yourself ?
you could have a corrupted fit file on your watch
get the corresponding fit file from within the activty folder on your device
run that fit file through Fit File Tools's section remover (whitout any removing)
import the resulting fit file manual from within Garmin Connect Web
when this fails
with your consent we can have a look at your file with FitFileRepairTool
zip the fit file and upload here in the forum (drag & drop on your post)
or provide a link to it on a cloud storage
happy & safe sporting
