The error is not from your bash script, it's from the shell wrapper script that Geany uses to execute your file (when you press F5 or select the _Build - > Execute_ menu item).
The default Geany _Execute_ action is `"./%f"` which takes the name of your file and wraps it in a script in `/tmp` like
#!/bin/sh
rm $0
cd '/home/username'
"./name of your file"
echo "
------------------
(program exited with code: $?)"
echo "Press return to continue"
#to be more compatible with shells like dash
dummy_var=""
read dummy_var
The error is coming from Line 7 of this script, i.e.
"./name of your file"
and is simply indicating that file `./install sauerbraten 2020` cannot be executed because of its permission bits - likely the executable bit is not set for your user. You can fix it with
chmod +x "./install sauerbraten 2020"