help me i have an admob error and i don't know how to fix it

i’m having this error saying "Assets/GoogleMobileAdsDemoScript.cs(39,27): error CS1061: ‘InterstitialAd’ does not contain a definition for ‘OnAdLeavingApplication’ and no accessible extension method ‘OnAdLeavingApplication’ accepting a first argument of type ‘InterstitialAd’ could be found (are you missing a using directive or an assembly reference?)
" plz help

OnAdLeavingApplication was removed in google mobile ads v6.0.0, I don’t know the reason but you can use google mobile ads v5.4.0: Release Google Mobile Ads Unity Plugin v5.4.0 · googleads/googleads-mobile-unity · GitHub

OnAdLeavingApplication was removed in google mobile ads v6.0.0, I don’t know the reason but you can use google mobile ads v5.4.0: Release Google Mobile Ads Unity Plugin v5.4.0 · googleads/googleads-mobile-unity · GitHub