By default in D365FO a form is not immediatly editable. If you want to edit you first have to click on “Edit” button in the top left of the screen (like shown below)
If you want a form to be immediatly editable you must open “Options” property and click on “Always open for editing”.
Next time you’ll open the same form this will be editable by default.
if you change your mind you can always go back and return to Options and click on “Default to system edit mode options”