Hi @edwardrc
Thanks for the screenshot.
I have seen users getting this error when trying to merge multispectral projects and you should follow the steps in https://support.pix4d.com/hc/en-us/articles/360038663932-Merging-multispectral-projects-in-Pix4Dmapper.
Maybe a quick win: unselect Duplicate Camera Parameters then try again.
Else (or if it is not a multispectral project), please provide the quality reports of each sub-project so that we can have a look and some information about what you are trying to achieve, thanks!