Pull the activity up in Garmin Connect, and do this in your web browser, not in GC Mobile. You'll go to a url like this to do that:

https://connect.garmin.com/modern/activity/1000000000

At the top-right corner of the page you'll probably see some things like "Sports" and "Health and Fitness" with blue or white text on a black background. Just below that, on the far right edge of the page, you should see a gray gear in front of a white background, it will be next to several other icons. Go ahead and click the gear. A menu will open up. The very last item in the menu is delete.

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Maybe this will be easier:

Go to this url:

https://connect.garmin.com/modern/activities

Click the checkbox next to the activity you recorded (driving to the store), and then click the Delete link near the top of the page, above the "Activity Name" column heading.

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Hope this helps!