writing vs write on

writing is a noun but write on is not a noun.

Word	Noun	Adjective	Verb	Adverb
writing	Yes	No	No	No
write on	No	No	Yes	No

writing (noun)	write on (noun)
the activity of putting something in written form
the act of creating written works
the work of a writer; anything expressed in letters of the alphabet (especially when considered from the point of view of style and effect)
letters or symbols that are written or imprinted on a surface to represent the sounds or words of a language
(usually plural) the collected work of an author

writing (verb)	write on (verb)
	write about a particular topic