Odoo will not connect
Symptoms of a failed Odoo connection and their fixes, in order.
1The symptom
The dropdowns on the settings tabs are empty — no journals, no accounts, no warehouses — or every Odoo step fails at once across all the screens.
2Check the connection
Open Configuration and, from the Save button menu, choose Check Connection. The message you get is Odoo's real one, and it is what will point you in the right direction.
3Open Configuration
The link opens the screen directly in the Integration Dashboard.
4Check the URL and port
The full URL with https://, no path after the domain, and no trailing slash. Port 443 for any Odoo served over https.
5Check the database name
A wrong database name gives a connection error rather than a permission error, which misleads a lot of people. On Odoo Online the name is usually the subdomain itself.
6Check the account and its rights
If the connection succeeds but exactly one step fails, this is a rights problem, not a connection problem. See Which Odoo account to use, and make sure the user is an Internal User.
7A sleeping Odoo
Trial databases and some hosting plans go to sleep after a period of inactivity, so the first connection fails and the next succeeds. Open Odoo in your browser to wake it, then check again.
8A firewall or IP allow-list
If your Odoo sits behind a firewall that only allows specific addresses, ask support for the connector server address so it can be allow-listed.