weapon change macroFollow

I’m creating a simple macro to change weapons.
When I try to change from sword & shield to 2 handed I’m getting an error message.
The macro I’m using is:

/equipslot 16 “name of 2handedweapon”

The message states there is no slot available.

How should the macro be written to replace a sword & shield with a 2 handed weapon.

Thanks.