Chances are that in the full error you will see something like
The located assembly's manifest definition does not match the
assembly reference. (Exception from HRESULT: 0x80131040)
When I experience a similar problem and run a unit test in the debugger then I see this sort of thing in the unit test output Pre-bind state information
Pre-bind state information
LOG: DisplayName = Microsoft.Data.SqlClient, Version=2.0.20168.4, Culture=neutral, PublicKeyToken=23ec7fc2d6eaa4a5
(Fully-specified)
LOG: Appbase = file:///C:/Dev2/MyApp/UnitTests/bin/Debug
LOG: Initial PrivatePath = NULL
Calling assembly : MyApp, Version=1.0.0.2, Culture=neutral, PublicKeyToken=c.
===
LOG: This bind starts in default load context.
LOG: Using application configuration file: C:\Dev2\myapp\UnitTests.dll.config
LOG: Using host configuration file:
LOG: Using machine configuration file from C:\Windows\Microsoft.NET\Framework64\v4.0.30319\config\machine.config.
LOG: Redirect found in application configuration file: 2.0.20168.4 redirected to 1.12.20106.1.
LOG: Post-policy reference: Microsoft.Data.SqlClient, Version=1.12.20106.1, Culture=neutral, PublicKeyToken=23ec7fc2d6eaa4a5
LOG: Attempting download of new URL file:///C:/Dev2/MyApp/UnitTests/bin/Debug/Microsoft.Data.SqlClient.DLL.
WRN: Comparing the assembly name resulted in the mismatch: Major Version
ERR: Failed to complete setup of assembly (hr = 0x80131040). Probing terminated.
The issue is an assembly binding issue the answer here may help. Don't worry that it mentions VB6 the actual issue is independent of that.