Have you ever applied for work permit, study permit, any type of visa or anything?
Like the answer in provided link says:
This eight digit number appears on all documents you get from Citizenship and Immigration Canada (CIC). It looks like this: 0000-0000.
If you applied to CIC before
Look for this number on:
letters you received from CIC,
your immigration document such as a PR card, study or work permit.
If you do not have any of these documents but want to fill out an application form:
you can leave the field for UCI (Client ID) blank.
Note: If you have not dealt with CIC since 1973, you will not have a UCI or a Client ID.
If you are applying to CIC for the first time
You will not yet have a client ID. If so, and you are filling out a form that asks for one, write “Not Applicable” or “N/A” in the space provided.
If you are applying online and the form does not allow you to input “N/A,” leave the space blank.