If you have T5008 slips or imported them using the CRA AutoFill My Return feature, quite often the amounts in Box 20 - Cost or Book Value is missing from these slips.
It is up to the taxpayer to complete this information so they are not paying Capital Gains on the entire Proceeds of Disposition. You may have received information about the purchase of these shares from your investment firm or institution that tells you what the Cost or Book Value is. This information is what is needed to be entered in a blank Box 20 of the T5008 slip.