Outlook: Cannot edit text while replying message.

I had to solve such issue for one of my colleagues before
The following microsoft page was helpful to troubleshoot this problem.

https://support.office.com/en-ca/article/I-can-t-change-the-formatting-of-a-message-when-replying-or-forwarding-f6c1b3ae-b949-4f91-b5a7-e3cf152b4590

Feel free to comment. Thank you all.