Mailspring doesn’t keep a local-only address book — contacts you create are written to your provider, so the New Contact button is only live when you’ve selected an account that has an editable address book on the server. Plain IMAP gives Mailspring mail only, no contacts API, so for that account you’ll just see Found in Mail (people you’ve corresponded with, which feeds autocomplete) and the button stays disabled with the tooltip “Select an account to add a contact.”
What makes an account editable:
Google accounts (via the People API — if you added the account years ago and it’s read-only, Preferences > Accounts > Re-authenticate grants the extra permission).
Any provider that offers CardDAV: iCloud, Fastmail, Yahoo, Posteo, Nextcloud and most hosted mail. Mailspring discovers this automatically for the account’s domain; when it finds an address book, an All Contacts entry appears under that account in the Contacts sidebar and you can add and edit there.