The following applies if you are using a version of the PDFTron software before IM Update 32.
The markup layers cannot be copied in Markup Layers Manager due to restrictions related to owning groups.
From Update 33 onwards, the owning group is not prepopulated on the Copy Markup File form in Markup Layers Manager. The default owning group value is set to NULL. For the latest information, see Using the Markup Layers Manager in PDFTron.