Since this is your first all-grain brew on the Anvil and you really don't know where your efficiency will come out don't sweat the actual numbers, especially since this is your firs tall-grain brew on the Anvil.
I am guessing that this is the Thin Mint Stout recipe on the BeerSmith Cloud.
The recipe on my end comes out with an OG of 1.096, so this is a high gravity brew. I have always expected my efficiency to drop as the gravity gets up over 1.080 or so and plan on that. At least in my opinion, this is pretty ambitious for your first all-grain attempt.
Having said that, anecdotal information from other people with the 10.5 Anvil have them being able to fit up to 19.5 lbs of grain into their Anvil with some work. You may need to change your mash volume versus sparge to get everything to fit right. The other aspect to look at is that of your fermentables, approximately 1 lb of that will be the lactose, which is a boil addition and not in the mash. So this gives a little relief on your grain bill for the mash.
Alternatively, you might want to consider would be to cold steep some of the roasted grains (roasted barley and chocolate malts) which will give you additional volume in the mash tun. The wort from the cold steeped grains can be added at the end of the mash.
Another option would be to cap the mash with these malts, so you would mash those malts which need conversion (brewer's malt, Vienna malt and flaked barley) and then add the dark grains at the end of the mash just before you do a mash out. My experience is that you don't usually get as much of the color or potential when doing this, but considering the low potential of the malts it might not make that much of a difference.
When I got my Anvil (6.5 gal), my first brew was a Scottish ale since that is a lot easier for the grain bill and more forgiving in the gravity.
Best of luck and let the community know if you have any other questions