You used a dependency that was built with .NET 4.0, if your project configuration is with .NET 2.0.. 3.5.

For Example : enter image description here

and config in project is .net 3.5 enter image description here

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To solve this problem, you should either move the .NET version of the project to version 4.0 or use a dependency with .NET version 3.5 or lower.

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