Archive hides a client from your lists but keeps every quote, job and invoice. You can find them again on the Archived tab.
Delete is permanent and only works for clients with no history.
If something's not working
"This client has quotes, jobs, or billing history — archive them instead."
Deleting would wipe their paperwork and payment records. Archive them instead: they disappear from your lists and nothing is lost.
I can't find a client I know is there.
Check the Archived and Contacts tabs. If you're in the Sales or Sales + Tech role, you only see clients assigned to you, so ask a manager to assign them.