The breakpoint will not currently be hit. No symbols have been loaded for this document in Unity

This is known bug in Unity. Here is how to fix this:

1.Go to File --> Build Settings... then select your platform.

2.Click on Player Settings --> Other Settings and change the API Compatibility Level to .NET 2.0.

If it is already set to .NET 2.0, change it to .NET 2.0 Subset then back to .NET 2.0.

3.Close and re-open Visual Studio. From Visual Studio, go to Build --> Clean Solution then Build --> Rebuild Solution.

Try debugging again and it should work.

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If the steps above did not work, Go your Project's Library folder then deltete it. Restart both Unity and Visual Studio.