Hello @FatFace68 and welcome 
Note: AUR targets mainly Archlinux Stable and Manjaro Unstable has the same package versions as Archlinux Stable. The script asks for libdrm_intel v2.4.109 which is only available at Manjaro Unstable.
If you really need Mesa Devel, then the solution would be:
- Switch to the unstable branch:
Switching Branches – Manjaro - Do a full upgrade:
pamac update --force-refresh --enable-downgrade --aur
Otherwise you need to downgrade it…