Can't overwrite .php files?

I was running some tests on the page.php file - but for some reason, any changes I am trying to save and overwrite.
It just doesn’t work.

Is there any kind of “lock” on this thing?
Something that prevents me to overwrite the existing .php file?

Hello,

Please check this WordPress Codex page:
https://codex.wordpress.org/I_Make_Changes_and_Nothing_Happens

Feel free to leave here a note if you need more help.

Kind Regards, Roman.