All the fields of the Application Form are filled but the “Check and submit” button still sends back that some sections are missing. What can I do ?

This issue may come from the multilingual fields, because some fields need to be filled for both languages.
As you can read in the Courtesy Application Form, part A.2, only the field “A.2 Project summary” must be filled in French and in English.
For the other fields, and only if the field is compulsory to be filled in, you can simply copy and paste the content of the field in one language into the field of the other language.