On Wikipedia it is said that Si vis pacem, para bellum means "If you want peace, prepare for war". But I think that It also seems like "If you want peace, prepare war". What makes these words to be distinguished with these two translations? (Because I think para = prepare?, here)
To see what nuances a Latin word has, a list of translations to another language like English is not quite enough. Examples, descriptions, and explanations help get a better picture. The link you give is better than a mere list, but the entry in this online version of the dictionary by Lewis and Short is even better.
The phrase para bellum can be well translated as "prepare for war", "prepare war", "provide war", "acquire war", and perhaps even more along these lines. It is unclear without context whether one should get ready for the possibility of war or actively seek war. The distinction cannot be made well enough with this verb, so both translations you suggest are valid. It is only the juxtaposition to si pacem vis (and perhaps further context) that suggests one should get ready in case a war breaks out.
The Romans used fewer words than ourselves to describe the same concepts. Therefore, "para bellum" may sound blunt/ terse to us; but, it is valid Latin, as Joonas has explained. The problem, here, is that the quote is incorrect. The original version can be found in some of the many "Latin Quotes/ Phrases/ Cliches"--type sites. The article on "Thought Co." is worth a look:
"igitur qui desiderat pacem, praeparet bellum" =
"Therefore, he who desires peace, should prepare for war."
[It was taken from the book: "Epitoma Rei Militaris" = "Examples of Military Matters", Liber III, 1, by Roman general, Vegetius (Publius Flavius Vegetius Renatus) late 4th. Century.]
The second verb, "praeparet" is a present-subjunctive--invoking the indefinite article--could/ would/ should/ might/ may; hence: "praeparet bellum" = "should prepare (for) war".
The nuanced meaning is a clear warning: a country wishing to live in peace, should develop strong armed-forces to dissuade potentially aggreessive neighbours from invading. Why did post-Communist Poland (and the Baltic States) rush to join NATO, while Russia was still weak? (Why didn't Ukraine do the same?) The "nuclear deterrent" is another example. A sad comment on the human condition, but valid nevertheless.
EDIT: 5/7/2020: Thanks to Seb (Comments): "si vis pacem..." is not so much "incorrect" as a more succinct alternative to Vegetius.