Main contact warning in pportunity

Hi pros,

What can cause a warning message "Main contact is initial. Delete it or enter an ID."

I am getting this message in the opportunity created with an individual customer. I can't understand what main contact has to do with individual customers. THis appears for every opportunity created from Lead with individual customer.

I ssee this message only in the test system which is in 1611, but not in the productive tennant with 1608.

Best regards,

Nadezhda Rukavishnikova