You had the same problem I am currently having, with that very same error.
My first Mosaic was a 12 panel about 3 years ago following Dave Aults method, it worked a treat, but now I'm in the process of a 6 panel and I've hit that wall with the error, I did have panels of differing sizes, 1-5 were the same with no.6 off, sorted that so now all 6 are the same but when I try to merge the same error that Terry had comes up.