If I select all this text inside Frame maker, I noticed that I cannot use the right arrow (it is greyed) in order to move the text to the right.
That heavy right arrow (🡆) is for Object alignment, not paragraphs.
You need to use the right para align button 1 row above and 2 icons to the right, or shortcut ␛jr
But this is not ideal document stewardship, as it merely applies an override to all the paragraphs, which will vanish when any of the formats are updated by various means.
This text was coming from a Microsoft Word document.
Sorry but I don't understand what button to use ?
If I try to modify something, FrameMaker says me : "modification impossible. Document for consultation only".
Is the file read-only or locked in some way?
It seems to be unlocked :
Did you check in File Explorer? Are you using Structured or Unstructured FM?
Meanwhile, I have deleted this document and created a new FM document by importing the same Word file inside, and it worked (I can modify again the file).
But I have always the problem of adding an offset to the text (move the selected text to the right). How to set the paragraph (see red arrow) in order to have this offset ?
Create a paragraph tag with the defined margins you want to have applied.
You don't seem to be very familiar with FM - maybe you should invest in a FM reference book like Matt R. Sullivan's Publishing Fundamentals?
Finally I have succeeded to create a paragraph tag with these settings (with an offset of 2 cm) :