You can only claim CCA for the current tax year on your 2020 Tax return. If you wish to claim CCA on the rental property in prior years you must file an adjustment to those years. For 2018 and after, you can use the REFILE feature in TurboTax to adjust your claim (2019 and later for Québec REFILE). For earlier years, you must use the form T1ADJ - T1 Adjustment Request (TP-1.R for Québec).
Note that if you choose to claim CCA on the rental property itself (building), you may be subject to CCA Recapture if you later sell the property, since real estate tends to increase in value, rather than decrease.
For more information on rental properties, please visit the Rental Income Centre.
For more information on adjusting prior year tax returns, please read, I have filed my tax return. Can I still make changes?