Upvote:0
Put a best estimate and add a note.
Did you read the instruction? Sometime they have some indication in case of high number of visits, or a temporal period (in last 10 years).
If you cannot trace them (so no stamp on passport), probably also they cannot get it.
Upvote:1
Delete all the rows and then add one again, I ran into the same issue today. Best of luck on your visa application.