This error means the information provided by the T5008 slip issuer is incomplete.
If this T5008 is for a disposition of shares/mutual units/etc., you must report it on your return. Enter your adjusted cost base in box 20 (or use the Capital Gains section, instead).
Money market fund or high interest savings account
If this T5008 is related to a money market fund/high interest savings account/etc. where there is a set unit price (e.g., $10 per unit):
Report the interest income you earned from this account using the corresponding T5 slip you should have received.
Decide whether you want to include the T5008 on your tax return. If you choose to include it, copy the amount from box 21 into box 20 (resulting in a gain of $0). In our experience, neither option (including or excluding these slips) triggers a review and the CRA does not have a stated position on the topic.