If this error is appearing in an IIS app such as ASP.NET, then there is a high probability that you are missing a binding redirect in the web.config.
When you install from Nuget, you should see a binding redirect in the web.config.
If you deploy the application to another machine but fail to set up the binding redirect on that secondary machine, then this error can appear.
<dependentAssembly>
<assemblyIdentity name="System.Memory" publicKeyToken="cc7b13ffcd2ddd51" culture="neutral" />
<bindingRedirect oldVersion="0.0.0.0-4.0.1.1" newVersion="4.0.1.1" />
</dependentAssembly>