Direct3D (D3D) Feature Level 11.1 is not a standalone download. It is a specific set of hardware and software capabilities integrated into the DirectX runtime that comes pre-installed with modern Windows operating systems Microsoft Support Core Requirements for Feature Level 11.1
If you believe your hardware is new enough, you need to ensure your software environment is correct. This process effectively "installs" the files necessary for 11.1. d3d feature level 11.1 download
If you have compatible hardware (e.g., a GTX 1060 or RX 580) and a game is demanding 11.1, follow these steps. You are not downloading the feature level itself, but the components required to activate it. Direct3D (D3D) Feature Level 11
: DirectX 11.1 is built into Windows 8, and 11.2 is built into Windows 8.1. If you have compatible hardware (e
If a game or app tells you it requires Feature Level 11.1, follow these steps to ensure your system is properly configured: DirectX Feature level 11.1 - Microsoft Q&A