As per instruction by IRCC if the personal does not have any last name or surname on passport then write full name in Last name and keep first name column black.In this case, in the given name section update your full name as it is on your passport and in the last name field just type LNU
This is a standard term for immigration. It stands for Last Name Unknown.
https://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=014&top=4