Missing required DLLs

Dynamic-link libraries are collections of code and resources that provide functionality for applications. Therefore, missing a DLL will often prevent you from using a Source program entirely.

Troubleshooting

A missing required DLL error is almost always caused by user error. However, it is generally easy to rectify. Here are some steps to do so:

Verify Integrity of Game Cache

This will almost always re-download the missing DLLs and is generally the solution.

Install DirectX Redist, all versions of Visual C++ and .NET Framework 4.x (4.8/4.8.1 recommended)

This would solves many issues with not only missing DLLs, but also game crashes related to DirectX 8/9 files not installed or it is outdated. This is also included from Steamworks Shared.

Take DLLs from other games

Sometimes a required DLL will not be part of the Steam depot for a particular game -- this tends to only apply to DLLs used by obscure tools that you weren't exactly intended to use in the first place. Since the "missing required DLL" error tells you which DLL it failed to find, you can rummage through the bin folders of other games you own in an attempt to find the missing DLL. Often these scavenged DLLs will not work because of binary incompatibility, but sometimes you get lucky.