For the current NuGet System.Memory package with version 4.6.0 in NuGet, you need to add this reddirect in web.config.

<?xml version="1.0" encoding="utf-8"?>
<configuration> 
    <runtime>
        <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">          
            <dependentAssembly>
                <assemblyIdentity name="System.Memory" publicKeyToken="cc7b13ffcd2ddd51" culture="neutral" />
                <bindingRedirect oldVersion="0.0.0.0-4.0.2.0" newVersion="4.0.2.0" />
            </dependentAssembly>
        </assemblyBinding>
    </runtime>
</configuration>

That's because in Project References, the NuGet package shows with this version 4.0.2.0.