The messages you will actually see

Messages mentioning journal, account, or "no account was set":

Meaning — an accounting setting is missing for the payment method or shipping company on this particular order. Where to fix it — the Accounting or Customer tab, especially the per-shipping-company and per-payment-method tables. The classic mistake is setting the default and forgetting a row in the table.

Messages mentioning lock date or a locked period:

Meaning — you are writing with a date inside a period your accountant closed in Odoo. Where to fix it — either open the period in Odoo, or change Create Invoice Date on the Accounting tab to use today's date instead of the order date. For cancellations specifically, switch Cancel Invoice Type to credit note.

Messages mentioning not enough or available quantity:

Meaning — there is not enough stock in the location the connector uses. Where to fix it — enter the stock in Odoo, or correct the Stock Location on the Inventory tab if the goods live somewhere else.

Messages mentioning access, not allowed or AccessError:

Meaning — the Odoo account the connector uses has no rights on that model. Where to fix it — in Odoo, grant the user rights on the named module. See "Which Odoo account to use".

Messages mentioning company or a company mismatch:

Meaning — the item, journal or account belongs to a different company from the one selected in the Odoo Configuration panel. Where to fix it — correct the company in the settings panel, or move the element to the right company in Odoo.

Messages about an item not being found when you can see it in Odoo:

Meaning — the item is archived (active = false), so the connector's search does not see it. Where to fix it — unarchive the item in Odoo, or map the Salla product to a different, active item.