HDR Pano unable to merge

This seems still be a problem (or, a feature which still has its limitations).  I really wish Adobe coders would make HDR panorama feature more robust. Software should be able to take uneven exposure brackets and perform HDR operation to merge panorama. 

That said, seems normal panorama feature, when you feed it different exposures, can do some exposure balancing which often is good enough.

Edit in photoshop then merge.

have you tried with (not) renaming the files?

that worked for me on a -1 0 +1 HDR pano with adapted Nikkor lens on Fujifilm GFX 50R

On my X-T3, have experienced that Photo Merge HDR-Panoramic is buggy. Too many frames, no go (think It worked for 90 or so but not past 96). Oh, this is not the exact error as the OP listed. But doing this outside of LR, in ACR worked.

And for even some small sets, for example 28 frames (7 exposure 1 stop bracket, 4 columns) I get that specific error, despite in camera auto bracket, 1 stop, 7 frames, bracketing order -0+. If I Romberg correctly, when I got that error and then tried using ACR instead, I git the same error.

so, truly odd, same camera, same in camera settings, sometimes works, more often does not.

The brackets have to be the same size and the exposures have to be in the same order. You can’t have -1 0 +1 and then +1 0 -1. It’s best to use auto-brackets.