Work down this list. Most problems are one of the first three.

1. Portal access is off for that person. Open the client in CRM, find the contact, and check the ⋯ menu: if it says Enable access, access is off.
2. The whole client is archived. Archiving turns off portal access for everyone at that company. Restore it from CRM → Archived. See Archiving and deleting clients.
3. Their invitation expired. Invitations last 24 hours. Use Resend invite on the contact and they'll get a fresh link.
4. They're using the wrong address. Check your portal address (yourslug.orbinly.com, or your custom domain) in Settings → Branding.
5. They're signing in with a different email than the one on their contact record. Sign-in links only go to the address you invited.
6. They have no role (on plans with roles). They can sign in but see no billing or work content until you assign one. See Contact roles and permissions.
7. An app is hidden from them by a tag. See Showing different apps to different clients.

Still stuck? Send them How clients sign in, or contact support.