You have Studio 2021 SR1 CU7 or CU8 installed, and when you are trying to open a Markdown (.md) file for translation, you receive the error message below:

Could not load file or assembly 'System.IO.Abstractions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=96bf224d23c42e59' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)

1. Please uninstall your current version of Trados Studio 2021 as described in this article: https://gateway.sdl.com/apex/communityknowledge?articleName=000012376

2. Install the latest version, CU 12, from here: https://gateway.sdl.com/apex/communityknowledge?articleName=000012378 
Double click on the msi file.

3. Open the Markdown file for translation

The issue has been fixed in the latest version of Trados Studio 2021.