I ran into the same error and fixed it by making some minor changes in the synth.ado. The conformability error seems to be caused by a non-critical command that can be captured. Here is what I did to make it work:
1) Go into the ado folder where Stata saves the packages and find the synth.ado file (should be in the folder ado/plus/s)
2) Open the synth.ado and replace line 3 "program synth , eclass" with "program synth2 , eclass"
3) go to line 672 and replace "mat rowname `wsolout' = `colabels'" with "capture mat rowname `wsolout' = `colabels'"
4) save the file as synth2.ado
Now you can run the synthetic control group with the same options using the command "synth2" instead of the original "synth".