You have script code inside of your submenu "Advanced options for Ubuntu". This is not valid syntax for grub. It's issued by a bad behavior of grub customizer - it shouldn't move script code into submenus.
You can fix it this way:
- show placeholders (View -> show placeholders)
- select the entry named "(script code)" inside of the submenu
- move it out of the submenu
When you save the configuration, you shouldn't get the error message anymore.
You have script code inside of your submenu "Advanced options for Ubuntu". This is not valid syntax for grub. It's issued by a bad behavior of grub customizer - it shouldn't move script code into submenus.
You can fix it this way:
- show placeholders (View -> show placeholders)
- select the entry named "(script code)" inside of the submenu
- move it out of the submenu
When you save the configuration, you shouldn't get the error message anymore.