Some information

Operating system: Win 10
Java version: latest, 16 or something I believe.
Minecraft version: 1.16.5
Baritone version: 1.6.3
Other mods (if used):

Exception, error or logs

Ingame feedback when doing "#set ca"

[Baritone] Filling now
[Baritone] Missing materials for at least
[Baritone] 101x null
[Baritone] Unable to do it. Pausing. resume to resume, cancel to cancel.

How to reproduce

Just mark an area with #sel and run #sel ca.

Modified settings

buildInLayers=true
layerOrder=true

Tried to reproduce and it works fine. Maybe provide more detail in form of a video
On the same version as you described.
(#set is supposed to be for settlings, I'm assuming you made a typo under Exception, error or logs)

Could you maybe include the log, or are you sure it does not contain relevant information?
And a video might help as well in case the error is in some way caused by the blocks (not) being replaced.