I have updated my packages to the latest versions you'll need to install dependencies first there 2 for r-lang 3 for monetdb of which one is r-lang anyway, you also need base-devel installing first otherwise you won't be able to build anything.

https://aur.archlinux.org/packages/r-lang - 3.1.2

https://aur.archlinux.org/packages/monetdb - 11.19.3

You should be able to build MonetDB on Arch Linux via AUR ( Arch Linux User Repository) without issues you need the abs package installed ( Arch Linux Build System ) first.

Regards,

PS: https://wiki.archlinux.org/index.php/Arch_Build_System

Brian Hood