Strange error message when grouping layers

I’m using Synfig Studio on a Mac and I keep getting a pop-up with

Group Layers [list of layers]: get_canvas()!=subcanvas

when I try to group several layers at a time.

Welcome to the forum.
Does the group layers you want to group contains other groups that are open? Then close all open group layers in the layers panel, select all groups you want to group and try to group them afterwards. (I get paid for using the word ‘group’ :smiley: )
That works for me on Windows.