Problems with Microsoft.directx.directinput.dll are somehow caused by a problem with Microsoft DirectX.

Follow these steps in the order they are described here to tackle the simpler solutions first.

Restart your computer if you haven't already. The microsoft.directx.directinput.dll error could be a fluke and a simple restart could fix it completely.

Install the latest version of Microsoft DirectX. Upgrading to the latest version is likely to fix the microsoft.directx.directinput.dll not found error.