FCPX not recognizing MOV files

What codec is being used? Can Lightroom tell you that? MOV is a wrapper, not a codec, and thus it can be a wide variety of codecs. There are outdated, legacy Quicktime wrappers that won't work on in FCPX, but if your version of Lightroom is up to date, that shouldn't be an issue. Can you share this file with us, via dropbox or wetransfer or something?

Not exactly what you asked but, if you can't get your normal workflow working, an alternative (and in my opinion, a much better) way to produce timelapse videos is to import the stills into FCP, add them to a timeline, make the duration of each still a single frame and off you go. You have a timelapse video which you can grade further in FCP. This has the advantage that you can export as a 10-bit (or higher) video such as ProRes or H.265 so you have a lot more control over the quality of the end product.

The stills format can be raw if FCP will read that (depends on the camera) or you can export DNGs, TIFF, PNG, JPEG-XL or other higher bit format that FCP can read from Lightroom.

And you can also make it HDR video if you wish and have a suitable monitor.