OCR attempts to recognize images of textual characters and provide an output glyph. String these together for words, sentences, paragraphs, etc.
So, this output is "renderable". OCR does not "do" renderable text - just the image of text. You have a text not an image of text (such as from an output image of a scanner). Acrobat is just letting you know that you've asked it to do what cannot be done.
So, maybe if you change the keyboard mapping from English to French (if the OS supports) then you can use the tool that lets you do minor edits to text to select and change characters to the French character.
n.b., The tool's name and "click path" to get it going varies with the version of Acrobat.
Be well...