The assume-installed flag is a pacman specific one.
It won’t work with pamac. (I don’t think pamac has an equivalent flag)
The pencil package depends on yarn and that one on nodejs.
So I assume you do not have yarn installed atm?
First install yarn, assuming nodejs is there and then install pencil:
sudo pacman -S yarn --assume-installed nodejs=12.14.1
pamac build pencil
However, I’d recommend to use the nodejs package provided by the repository rather than the “manually installed” one if possible.