I would like to add laps for my indoor rowing activities. I see no possibiltiy to add it in Garmin Connect web or Android app. There is only one lap.
You can add laps manually during the activity, by pressing the lap button. On many Garmin devices the lap button is by default disabled during the activity, so you have to enter the settings of the respective activity and enable the lap-button function.
Another way of creating laps is turning on the auto-lap feature. Again, you have to enable it in the settings of the concerned activity. However, on some devices only distance based auto-laps are available, which would not probably work at indoor rowing.
I would like to add labs for my indoor rowing activities. I see no possibiltiy to add it in Garmin Connect web or Android app.
As I wrote, you have to set the laps with the help of the lap button during the activity. If you want to see separate statistics for certain sections of your rowing activity after the the activity was saved (without having used the lap button), use the trim function. It is a reversible feature, so you can trim the activity as often as you wish. Trim for example a section from the 10th minute to the 20th minute, to see the second lap of 10 minutes.
but I would like to edit/add the labs afterwards because my rowing machine cannot sync with my watch or computer.
You cannot, there is no such possibility. You can only enter laps on the watch. Or you can use the Trim function to view stats over selected sections of the workout, and then reverse the trmming.
If you are importing the activity data from the rowing machine, then you could edit the file (assuming it is in a human readable format) and insert the laps there.