I really don't know anything about how standard that header is, as I've only learned its existence when seeing that error I'm getting from my Python scripts. I solved it by creating the header with the value "adu", so that's what worked for me.

Here is the paragraph about this header in the FITS standard:

STATUS:    reserved
HDU:       image
VALUE:     string
COMMENT:   physical units of the array values
DEFINITION: The value field shall contain a character string,
describing the physical units in which the quantities in the array,
after application of BSCALE and BZERO, are expressed.   The units of
all FITS header keyword values, with the exception of measurements of
angles, should conform with the recommendations in the IAU Style
Manual. For angular measurements given as floating point values and
specified with reserved keywords, degrees are the recommended units
(with the units, if specified, given as 'deg').

My understanding is that the value would pretty much always be "adu" for all uses specific to consumer-grade astronomical cameras. Maybe in some research environment a camera can be directly counting individual photons, but for what we all do here it should be "adu".