So basically exactly what the title says. I have an item. I based it off of a musket, so it's a 1d12. I want it to have versatile 2d6. 

I go to add modifier. Select Weapon Property. Select Versatile. Leave the stat blank (or do dex, it doesn't matter same result) I do 2 for the dice count, d6 for the dice size. I then save it go to any of my characters add the item and it shows versatile but the damage die is a d20+modifier instead of 2d6+modifier. Why? I don't understand how what i selected above is incorrect? 

A musket is a gun. The versatile rule is for melee weapon’s only. In addition, versatile doesn’t work that way.

Versatile raises the die size by one. So a 1d6 was ill always become a 1d8. A 1d8 will always become a 1d10. The only die higher than a 1d12 in D&D is the d20, so adding versatile to a 1d12 will always make it a 1d20.

I hope this has been helpful.

So is there not a way at all to have two different damage ranges on a weapon on dndbeyond? Is the Solution just to do a custom action?

You're right, versatile the way it is written doesn't work that way because it is designed to be more damage if used with two hands. It was simply the closest thing I could think of to have two different damage ranges. I'm playing an artillerist artificer /  fighter multiclass and after tomorrow will be able to take arcane archer. My DM instructed me to make a custom gun/crossbow combo weapon with two damage ranges. 

Quote from XBguile >>

You're right, versatile the way it is written doesn't work that way because it is designed to be more damage if used with two hands. It was simply the closest thing I could think of to have two different damage ranges. I'm playing an artillerist artificer /  fighter multiclass and after tomorrow will be able to take arcane archer. My DM instructed me to make a custom gun/crossbow combo weapon with two damage ranges. 

 

 Is it supposed to be either/or, or added together?

One or the other. Essentially my arcane shots would be shot through the crossbow, while my regular shots would be through the gun, but it's the same weapon. Just a flavor thing really and he wanted me to use unique damage numbers for it as I did a bunch of work on the lore for the weapon and what not. 

You’re going to have to use custom actions then. There’s just no way to do what you want as there are no ranged weapons that work that way. Good luck.