Error on 404 page

On the page it is possible to set your own error page so-called Error Page. The site works, for example, majonetwork.page.tl/tralalallalal.htm
But when the address changes into majonetwork.page.tl/tralalallalal the error page does not pop up with the usual error page code, only the page does not exist, etc. So if you do not enter the .htm tag which means what the page was written on page error page is corrupted I am waiting for a quick fix of this error, it is not fatal but you have to bury it in the home page and editor files. Yours sincerely, Majo Network.