There are multiple things to consider here
This popup might come at multiple places when you are filling each line with serial number
So use a PARALLEL ACTIVITY and inside that
In left side keep all the activities you want to perform like typing serial number in each line or so
Then in right side of parallel activity use a sequence block where have a element exists activity and check for this popup
Get the output as bool_exists
Inside the same sequence use a if activity and mention the condition as
bool_exists = True
If true it will go to then block where use a click activity to click that on button
For more details on parallel activity
Cheers @Dhanushka_Gunasinghe