Techtalk.Specflow error: Could not load assembly file or assembly

Check your package.config and see if it has an entry for TechTalk.SpecFlow. If yes, then delete the entry from package config file ... as well delete all the assembly related to that dll. Re-install the specific version from NuGet and re-compile your project.

Per your comment it still targeting the new version newVersion="2.1.0.0" and not the new version Version=1.9.0.77. Change your app.config like to be

<assemblyIdentity name="TechTalk.SpecFlow" publicKeyToken="0778194805d6db41" culture="neutral" /> 
<bindingRedirect oldVersion="0.0.0.0-2.1.0.0" newVersion="1.9.0.77" />