If auto-post is on, Link My Books holds any settlement with a line that has no account or tax rate mapped, so it isn't posted with gaps. You'll see this message on the settlement and a notification in the bell:
"This settlement is on hold because some of its lines have no account or tax rate mapped. It cannot be auto-posted until they are mapped in Account & Tax Mappings."
To fix it
Go to Account & Tax Mappings, map the missing account or tax rate, and save. The hold lifts by itself once everything is mapped, and you'll see "Hold released: account and tax mappings are now complete."
Once the hold is released, the settlement goes back into auto-post as normal. It still has to fall inside your auto-post settings (for example, the date you turned auto-post on from) before it posts.