Some pictures from an iPhone have a "RunTimesincePowerUp" tag:
exiftool -s -a -G1 -struct .\IMG_0877.jpg
...
[Composite] RunTimeSincePowerUp : 0:23:14
but when I use:
exiftool -composite:RunTimeSincePowerUp= .\IMG_0877.jpg
I get this output:
Warning: Sorry, composite:RunTimeSincePowerUp doesn't exist or isn't writable
Nothing to do.
How can I remove this tag if it "isn't writable"? Do I need to look for a different tool?
Composite tags (https://exiftool.org/TagNames/Composite.html) are not tags that exist in the file. They are created on the fly by exiftool based upon other tags in the file. The only way to change them is to edit the underlying tags.
In this case, RunTimeSincePowerUp is based upon two other tags, Apple:RunTimeValue and Apple:RunTimeScale (see Apple RunTime Tags (https://exiftool.org/TagNames/Apple.html#RunTime)).
Unfortunately, neither of these tags are editable by exiftool. They are also so very specialized/obscure that is very unlikely that you'll find a program that can edit them.