You should see BuyingGuideLabel.Text resource key under:
Local Resources\DesktopModules\Revindex.Dnn.RevindexStorefront\Portals\_default\Display\ProductDetail\StandardRegular\App_LocalResources\Display.cshtml
If you're using a custom template, it would be located in the same path except replacing _default with your portal number.
Are you changing it from a different place?
if it's missing the BuyingGuideLabel.Text resource key, you can try adding it to the resource file.
I think you may have said you tried this so I suspect you may be running a custom template but maybe just not aware of it.
One way is if you find custom templates (see the path under your portal number instead of _default path), try adding a dummy HTML text (e.g. "Hello world" into the cshtml file and see if it prints on screen for the product.
Custom templates can be applied at the product level, at the module setting level and under the Configuration > Product detail level with each one overriding the setting of the other in their respective order.
I hope this helps.