I think others already figured out SetClrVersion is a function of some West Windo component, maybe the DotNetBridge? Because from the top of my hat, using that bridge one of the first things to do is decide for the .NET CLR version to use, i.e. which .NET Framewwork version.
Anyway, when you do this in VFPa 64bit you also have to have a 64bit DLL, as simple as that.
I think I remember some posts of Rick Strahl saying he's not tested West Wind components with VFPa, so expecially not 64bit, but I'm not even sure the components West Wind offers for VFP are available in 64bit versions so they could be used (at your own risk) with VFPa 64bit at all. I know there's also a 32bit VFPA version. But of course, when you went for VFPA specifically to enter the 64bit world, you wouldn't benefit from reverting to 32bit, though quite a handful of fixes are applied that make VFPA 32bit advanced without going to 64bit, too.
Well, and all that said, sending mails is possible without needing to go to dotnet. So if you actually don't use .NET you still have very many options for sending mails with and without 3rd party components. So if switching to 64bit is more important to you than keeping bakwards compatibility, why not switch to anything else for sending mails? Blat is one third party component that's also available in 64bit.