I withdrew a lump sum from RRSP in 2020. I didn't receive the slip. How do I enter this amount in "income" in turbotax?

The financial institution from where you withdrew your RRSP should have provided you with a T4RSP. You should contact them and ask how you can get a copy.  

You may also be able to view your T4RSP by logging into the Canada Revenue Agency (CRA)’s My Account service. You can log into My Account on this Canada Revenue Agency (CRA) page: My Account for Individuals

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