There is no place to screw in screws for the right joycon sensor rail.

I ran into the same situation when my granddaughter somehow managed to rip one of her joycon controllers off the Switch, leaving it in the same situation you're in, with nowhere to screw in the rail.

I looked at several different possible solutions initially, but finally ended up biting the bullet and replacing the midframe. The midframe is what the joycon rails screw into, so when they're broken out the pot metal the midframe is made of tends to come apart. Here's the guide I ended up writing as a result of that adventure.

Nintendo Switch Midframe Replacement - iFixit Repair Guide

The other repairs I considered included things like filling the broken metal with JB Weld epoxy then drilling and tapping new holes, buying longer screws with nuts to be mounted behind the original screw mount, or just epoxying the screws in permanently.

To be honest, there isn't really a good solution here; the one I picked means you have to completely gut the entire Switch and move every single component to a new midframe. However, the other choices just didn't strike me as much better in terms of being a long-term permanent solution that was going to stand up to my rather breakage-prone granddaughter.

That's what I ended up doing; if you decide on another solution I'd love to hear about it; take pictures and maybe make your own guide to that repair; I would sure love to have had a better choice available. Let us know how it all turns out!