Panorama creates images with pixel-errors

to rule out corrupted source files (visually, it was my 1st assumption), try to open the original, inspect them for being okay and save them under new names. then try a new stitch.
another possibility is a huge parallax error as you might have stot these images freehand. you've a strong foreground/background relation and image depth where the parallax error might render neighbored images unstichable as either the foreground matches or the background, but not both. the adobe stitching algorithm is known to not be very robust against such errors.
what may help is to export both images from LR as layers into a PS file and then try to align them first and then blend them together. while it's more or less the same photo merge process in LR and PS, the split 2-step procedure in PS with aligning and blending has advantages with problematic material compared to the one-shot process in LR. you have a bit more control.

You need to give more information. What Mac hardware are you using (especially what GPU does it have)? What version of MacOS? What version of Lightroom?