Hello Giovanni Colombo,  
this is bacause the template was not updated to work with Joomla 4.x.
You need to update all files by hand and remove deprecated functions or replace them with the new ones from J4.  
Alternatively, you have to wait until joomshaper updates the template for J4.
I'm sorry, I don't have any better news.
-> https://www.joomshaper.com/joomla-templates/indigo -> Changelog
Version 2.0.0
21 October 2022
Improvement
- Latest version of Joomla 3 - v3.10.11
 
- Later version of SP Page Builder 4.0.4
 
- Latest version of Helix Ultimate 2.0.11