I've a Garmin fenix 6 watch and was wondering can I use my watch as power meter if I connect my watch garmin bike sensors ?
There may be some confusion with terminology here. A power meter measures power, usually by measuring the strain in some drivetrain component (i.e. how much it flexes when pedaling). You need to physically attach strain gauges to the component. This is usually a crankarm or pedal spindle, but strain gauges have also been mounted to bottom bracket spindles or inside rear hubs. The Fenix 6 is a watch, and it's nowhere near the drivetrain, so it can't be a power meter. It can track heart rate by itself, and heart rate can also be used to pace a workout. (see footnote)
Power meters then broadcast the torque measurements to a head unit, usually a cycling computer we put on our handlebars (or, if you are a track cyclist where they are banned, you put them on the back of your seatpost). They usually broadcast over two protocols, ANT+ and Bluetooth (older meters may only use ANT+). In any case, the Fenix 6 appears to have ANT+ compatibility, as do a few other sport-specific smart watches. Generic smart watches like the Apple Watch do not have ANT+, so you can't use them as head units capable of displaying power (although most should be able to pair to a Bluetooth-capable heart rate strap). The ANT+ page for the Fenix 6 Solar, European model lists many power meters, including power pedals by Garmin and Favero, crank-based power meters from Quarq, Shimano, Rotor, and Easton, and, as an aside, One Giant Leap's Kayak Power Meter (to emphasize, this is for kayaks, as in the ones you use on water). Thus, you can use your Fenix 6 as a head unit if you don't want to buy a dedicated one. It should be able to pair to power meters in general.
Occasionally with technology, the devil is in the details. One Reddit post claimed that their Fenix failed to display the cycling dynamics (an advanced metric) from their Favero power meter pedals. There may be other relatively minor issues like this with other manufacturers. As a different example, Garmin's Varia rear facing radar warning system transmits over ANT+, and requires a specific software protocol. Wahoo very recently implemented the protocol, so now I can see the traffic warnings on my head unit, but Wahoo failed to let users see the battery's state of charge or the light's flashing mode. It could be that the Fenix lacks some implementation of one of the features on some power meters, but the core features (it will need both torque and cadence) would likely be present.
The OP asked if the watch could be used as a power meter if they connected it to unspecified Garmin bike sensors. The Fenix 6 is an impressive unit. However, as explained above, it can't read power by itself. Unless one of the other Garmin sensors was a power meter (e.g. the Garmin vector power meter pedals), the answer must be no. If you want to use your Fenix 6 as a cycling head unit, you would want to get a bike mount. If you are primarily a cyclist, I would normally recommend just getting a cycling-specific head unit, which will have a larger screen and cost less (perhaps a lot less) than the Fenix 6. However, the OP already has a Fenix 6 watch, and it would obviously be fine to reuse it as a head unit.
Footnote: other ways to measure power
It's possible to estimate power from air resistance, e.g. the Notio Konnect or the Power Pod. This requires a device with a pitot tube at the front of the bike. It's also possible to estimate power from chain tension. No active power meters do this, but Polar once made such a unit. In any case, the Fenix lacks any functionality that would get at either of these methods.