ネットの情報が正確とは限らないって英語でなんて言うの？

"You cannot always believe what you read on the internet."

Always - something that is done or said every day or consistently. For example

(I always take the garbage out in the morning)

"Everything on the internet is not accurate."
Everything - news, sports, photos, etc.

Accurate - meaning that every detail may not be right or that there may be lies.

"Don't believe everything you hear/read!" - Common expression with a literal meaning. As in "Don't believe everything you read online, especially with all this fake news flying about!"

"You've got to take it with a pinch of salt." - Quite a nice phrase 'a pinch of salt' being not to take things seriously. As in "You've go to take everything off that website with a pinch of salt, non of it's true!"

In example three I use the phrase "with a pinch of salt", this means to not be completed convinced of or believe something that you hear or read, because it may be untrue or even because it sounds unlikely. For example; "Everything he says needs to be taken with a pinch of salt, as he tends to romanticize certain topics.

False - untrue or fake

Both sentences are great ways to say that information on the internet isn't always true. Which sentence you use depends on your personal preference.

"Don't believe everything you read on the internet"
Is a common statement, explaining that you shouldn't trust everything you may read on the internet.
"Not everything on the internet is accurate"
Again referring to everything on the internet is not accurate/true.
'Accurate' is a term used when referring to something that is true.

When you want to explain that the things you read on the internet is not always true; then you may say this in the following ways:
-What you read online isn't always necessarily true
-What you read on the internet is not always true