There are four Canon RF Macro lenses that can be used on an R6 without need for any adapter, however only one of them is capable of full 1:1 magnification... the RF 100mm f/2.8 IS USM (which actually can do 1.4:1 magnification, greater than lifesize).
The RF 85mm f/2 IS STM and RF 35mm f/1.8 IS STM, as well as a new RF 24mm f/1.8 IS STM that's coming to stores soon all can do 1:2 or one half life size on their own. They all could be pushed to higher magnification by adding some macro extension tubes behind them. Canon doesn't yet make any macro extension tubes for the RF mount, but other manufacturers do. In fact, Kenko has announced a set that will be in store soon.
You mention having gotten an adapter with the camera and lens. You may want to keep the adapter because there are a great many EF lenses that can be adapted for use on the R6, including quite a few macro lenses. Over the years there have been many very good macro lenses made for the EF mount, both by Canon themselves and by third party manufacturers. Currently there are only four autofocus macro lenses in production... a Canon EF 100mm f/2.8L IS USM, one Tokina and two Sigma. They all are pretty darned good to excellent. But if you shop used you will find a great deal more, including a number of different Canon macro lenses that were made until just recently.
Return the EF-M lens (as noted by Ricky, it cannot be adapted for use on the R6).
While it's possible to adapt them for use on your R6, I also would advise against EF-S lenses because the R6 is a full frame camera and EF-S lenses are designed for use on crop sensor cameras. EF-S lenses won't fully cover your camera's larger sensor, so images will be significantly cropped.
Canon EF lenses (no S or M) are made for full frame. There also have been 3rd party lenses made both for full frame and for crop cameras. For example, Tokina designates their full frame capable lenses FX and their crop only lenses DX. Sigma designates full frame DG and crop only DC. Tamron designated their full frame lenses "Di" and their crop only lenses "Di II". In any case, you should try to stick with full frame capable lenses.
Finally, there are also a number of manual focus/manual aperture 3rd party macro lenses being offered that will directly fit onto the R6. I mention these because some people prefer to shoot macro with manual focus.
If this is your first and only macro lens, I recommend a 90, 100 or 105mm as a good all-around choice. Longer focal lengths are tougher to hold steady, more likely to need a tripod and render very shallow depth of field. Shorter focal lengths can put you awfully close to your subject. So unless you have special need, I'd suggest the 90 to 105mm range as a good choice.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & ZENFOLIO
