How to reproduce:
1. Open the user's attached project "Ads Creator Client export.zip"
2. Make sure all the Scenes are selected in the Build Settings
3. In the Project Settings navigate to Player > Publishing Settings and tick off the "Custom Keystore" option
4. Build the project to either an Android or iOS device

Expected result: Builds to Android without any errors
Actual result: Build fails with the following Console error: "IL2CPP error (no further information about what managed code was being converted is available)
Unity.IL2CPP.HashCodeCollisionException: Hash code collision on value `483E6DDCA003A8CEBD350443456C4F5FB7439B58`
Existing Item was : `System.Void"

Reproducible with: 2020.2.2f1, 2021.1.0b3, 2021.2.0a2
Could not test with: 2018.4.31f1, 2019.4.18f1 (multiple Console errors after downgrading, missing references that I couldn't fix)

Notes:
- Does not reproduce on Mono
- Reproduces on both target architectures (ARMv7, ARM64)
- Reproduces on MacOS when building to iOS (does not reproduce when building to standalone)
- Reproduce on Windows 10 when building to Android (does not reproduce when building to standalone)