In my case, I got the issue after upgrading to version 4.5.4 and tried @user2713341 answer. It didn’t work but put me in the right direction.
My project had no bindings for this library, so I added the binding and it worked
<dependentAssembly>
<assemblyIdentity name="System.Threading.Tasks.Extensions" publicKeyToken="cc7b13ffcd2ddd51" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-4.2.0.1" newVersion="4.2.0.1" />
</dependentAssembly>
and it worked.
Note that it should be 4.2.0.1 even though the version is 4.5.4.