On the workout type tap on the 3 dots top right corner and tap on"Open" then start a workout and when you've finished tap on "End",Scroll down to the bottom and tap on "Done" the watch will remember your last setting and next time you do a workout it should be set to open goal,if not you may have to do it again as the watch enters whatever goal you set most and uses this as a default.
Here is a post from another forum that fixed the problem for this lady;
What I did to fix it was start an "OPEN GOAL" outdoor walk, and walked vigorously for about 1/2 an hour and just hit end when I was done. It still had a 20 min time set in yellow when I was finished. But the next day the 20 minutes had disappeared and OPEN GOAL was back in green, so I am assuming that is how you fix it. It must reset to the last thing you did overnight.
If the above doesn't work here's plan B;
Make sure you are wearing the watch;
On the workout type with the unwanted goal,tap on the 3 dots in the top right hand corner and tap on "Open",then start the workout and let it run for at least a minute, then tap on "End".
Scroll down to the bottom and tap on "Done".
The watch will return to "Open Goal"
There is a small chance you may have to repeat this for it to kick in.