As @EdwinAshworth says, one cannot simply make up new words*, or at least one can't expect others to understand the results. English has, over the years, borrowed words and lexical rules from many other languages, resulting in the richly varied and often wildly inconsistent language we have today.
For example, we have at least five** different prefixes meaning "not": "un" ("unlikely"), "im" ("impossible") "a" ("apolitical") "non" ("nonpartisan") and "in" ("independent"). It is not the case that you can stick any of these at the start of any existing word to get a new "valid" word. You would have something that people would have to try to guess the meaning of. Some wouldn't bother, some would get it right, and some would get it wrong. All of them might think you can't speak English.
*Making up new words, or neologisms, does actually happen all the time. It can be fun. Some of them catch on and enter the dictionary. But, all of them tend to cause confusion in the reader or listener, and if your aim is to create a clear communication then you should avoid it.
**this might increase as I keep thinking of others and coming back to edit the question.
EDIT: as @tchrist points out in the comments, the in-, il-, im- and other similar (ie an i followed by a consonant) prefixes are all really attempts to spell the same prefix, which is simple "i-". That is, i'logical becomes "illogical", "i'possible" becomes "impossible", "i'credible" becomes "incredible", etc, simply according to which consonant seems to most naturally fill the gap represented by the apostrophe in my examples above. In other words, if one was to try to say "ih-possible" it's a bit awkward, and we will "pick" a sound to fill the gap, and "im" seems most natural in that case.