Why Compose blocks some networks
A gallery and a YouTube upload are different objects. The composer treats them that way.
Validation
Text, image, gallery, video and short vertical video each have a support flag per network. Unsupported targets are listed before publish, with a reason you can act on.
Reddit is picky about the title
A Reddit target needs a title even when the caption is already written. An empty community posts to the profile feed. A community name posts there, and a flair id is optional when that community requires one.