[Fixed] 'IServiceCollection' does not contain a definition for 'AddApplicationInsightsTelemetry' and no accessible extension method 'AddApplicationInsightsTelemetry' (are you missing a using directive or an assembly reference?)

For this, you need the following NuGet package.

.NET CLI 

dotnet add package Microsoft.ApplicationInsights.AspNetCore --version 2.20.0

Package Manager Console

Install-Package Microsoft.ApplicationInsights.AspNetCore -Version 2.20.0