The Publish window checks each post as you go. Under Post to, a network with a problem shows a warning sign instead of a tick. Tap that network's name above the text box to see its message under the text. The button at the bottom stays greyed out until every problem is fixed.
Messages and fixes
"Needs an image." Instagram and Pinterest need an image. Add one in the editor's Media tab, then select it under Media in the Publish window.
"Needs a video." TikTok and YouTube need a video. Export one in Videos, then in the editor's Media tab tap From library to attach it.
"[number] characters over the [limit] limit." Shorten the text for that network.
"[number] posts are over 280 characters." In an X thread, shorten the long paragraphs or split them.
"At most [number] files." Deselect some media.
"Add some text or media."
"One of the files no longer exists." Re-select the media.
Each network has its own text box, so you can shorten one without changing the others.