Thread necro, but same problem, and a way to resolve...
Get into the command prompt
go to the ~/RetroPie-Setup/scriptmodules/supplementary directory
Delete the old
retropie-manager.sh file
Download the correct script from the github repo (
https://raw.githubusercontent.com/RetroPie/RetroPie-Setup/master/scriptmodules/supplementary/retropie-manager.sh)
Go back to RetroPie setup again and now the script should work and install retropie-manager.
("code" is below if needed)
F4
cd ~/RetroPie-Setup/scriptmodules/supplementary
rm -r retropie-manager.sh
wget https://raw.githubusercontent.com/RetroPie/RetroPie-Setup/master/scriptmodules/supplementary/retropie-manager.sh