Anyway to replace the zdoom error/unknown sign's sprite

It means the sprite is missing from the file or the missing sprite needs replacing with something else.

Try making a .pk3 file with the blank file named "unkna0.png" under the sprites namespace. (That's the name of the sprite in question.)

no, i am trying to replace the sprite that appears when something Is missing.

Have you tried replacing it on the map editor? I get that image when the sprite is missing from the file so I open up the map editor and replace with the missing thing with something else.

I am trying to make an error hider mod so when, for example, an unknown actor appears in a map it will be replaced with a blank sprite.

Never heard of that one before. When I encounter that image in testing map, I know something is missing so I remove the missing thing or replace it. Does the actor have an ED number? Are the sprites all there in the file or something? Because has got to be missing or not registering. If not, then I have no idea what's causing that for you. I've only encountered that '!' image in testing/playing map when sprites are missing from the file.

open gzdoom.wad in (or zdoom.wad) in a lump editor, you should be able to find this exact sprite - replace with a diff sprite using the same name.  should be fine. 

No way around that, you'll have to replace unkna0.png in the Sprites folder.

Or, perhaps, create a new actor in Zscript.