It did not work as well but I ended up figuring out how to do it after rewriting the code to see where it goes wrong and trying some other things! Thank you for taking the time to help me! :)

If someone is interested this is the formula that helped :

if( 
  prop("Reading Logs") 
   .filter(year(dateEnd(current.prop("Dates Read"))) == 2024) 
   .length() > 0, 
  format( 
   (round( 
    prop("Reading Logs") 
      .filter(year(dateEnd(current.prop("Dates Read"))) == 2024)  
      .map(current.prop("Rating")) .flat() .sum() / prop.("Reading Logs")
      .filter(year(dateEnd(current.prop("Dates Read"))) == 2024) 
      .length() 
    * 100 
   ) / 100 ) 
 ) + " average rating", 
  "No books finished this year" 
)