They left out the second half of the sentence: 非 [not] 常 [eternal] 道 [way]

Classical Chinese is highly contextual, and characters take on different meanings depending on the position in the sentence and the narrative context, hence the second instance of 道 meaning "speak, expound" instead of "way, method".

My favorite instance of this is from the Zhuangzi: 物 [thing] 物 [thing] 而 [and/under those conditions] 不 [not] 物 [thing] 于 [among] 物 [things]

Which translates to "treat the things of the world as things, but don't be a thing among them" which means that you should use strategic thinking to avoid becoming a victim of circumstances.

This is not even getting to that; but that is actually pretty simple, to be fair.

The negation is: 非常道

非 -- fei, this has been a negator for a very long time in Chinese

常 -- chang, this means something like common, usual, normal, or nominal (I suspect that translating this as "eternal" is a creative move on the part of some westerner)

道 -- dao, the way again

Nowadays, 非常 as a compound is used to mean "extraordinary" ("not normal", "exceptional"); but it is not hard to see how it might be taken to negate 道 here.

Interestingly the 常 was originally 恒, meaning “constant,” in the oldest manuscripts. It got changed to 常 because it was taboo to write any character appearing in the given name of an Enperor, and Emperor Wen of Han had the given name 恒.