I don't know this "ftp" feature or how it works but that looks really strange if it is listed in the specs of the watch directly on the shop page and not supported ?? Maybe it is supported but not the way you expect? Otherwise that's very weird indeed.

Forerunner^® 255

and I can confirm it's listed as compatible, I copy pasted it above directly from the shop, weird.

No, I am sure it is not supported because Garmin itself said so in this thread: https://forums.garmin.com/sports-fitness/running-multisport/f/forerunner-255-series/301689/fr255-is-missing-features-related-to-cycling-ftp#pifragment-1279=1 .
Here Garmin (last post on the thread) clearly admits that the feature was not intended to be present on the 255, and that's why they have removed it from the manual (again, only thanks to that thread). But they still won't remove it from the store, as we can see by the screenshot you have posted.

There is a big section in the manual page 67, you confirm you can't do what is explained there on the watch? That's a bit crazy they would expose themselves to a class action like this.

www8.garmin.com/.../Forerunner_255_OM_EN-US.pdf

Just to clarify: The watch supports functional threshold power for running, but not for bicycling.

Page 67 of the manual doesn't show any feature regarding FTP (cycling functional threshold power), except from the possibility to set an already known FTP value on the watch so that the watch can calculate the power zones based on that. The autodetection part only talks about maximum hearth rate and LT (running lactate threshold).

That's because, as I said, months ago, after that thread I linked was created, Garmin removed the FTP functionalities, that were never present on the 255, from the manual. So they were in the manual when the 255 started selling, but they are not there anymore.

The problem is that the same thing has not happened to the shop page of the 255, where the FTP functionality is still shown, as if the 255 supported it.
And the worst part is that we know Garmin knows, because
1) that thread above mentioned it
2) I (and probably other people) have emailed Garmin to tell them about this, and they never replied back after I sent them the proof of this

Garmin is then liable to be sued in court and suffer the consequences of misleading advertising, even having removed the function from the manual, as it continues to publish on its website, media and advertisements that the Garmin Forerunner 255 watch has this function. According to the Brazilian Consumer Protection Code, misleading advertising is any type of information or communication of an advertising nature, wholly or partially false, or, in any other way, even by omission, capable of misleading the consumer regarding the nature, characteristics, quality, quantity, properties, origin, price and any other data about products and services.

So Garmin is misleading the consumer, that is, a false perception of reality, and the injured consumer may file a lawsuit and claim Moral Damages. Garmin then do something, implement the function or bring new functions as compensation, as I will not hesitate to sue the company if this does not happen.

Based on what you are telling, FTP (cycling) was never in the watch but was in the manual. To me, it's more an error than anything else then. And if FTP (running) is in the watch and the manual (and on store description) then it makes sense it still advertised. I'm probably missing something anyway.

FTP is only a cycling metric. LT is the running feature you are confusing with FTP, but they are two completely different things. There is no "running FTP" in any Garmin product. Plus FTP was not only in the manual, but also in the store and it is advertised in every single store you go to buy the watch because Garmin has officially claimed it to be a feature of the watch.
The feature is still advertised on the store even after multiple attempts from us to tell Garmin to remove it from there.
This could have been a mistake in the beginning, but now it is obviously deliberate.