This is a very common error message in Odoo

The most common reason for this error is that you haven’t installed a Chart of Accounts.

This is done during database creation (see below) or by selecting a “Fiscal Localization package” in the Accounting app.

This creates the chart of accounts and also sets the defaults for the Account Receivable and Account Payable used on partners and may setup Fiscal Positions.

• The “install more packages” button displays a list of modules for you to select.
• This will either be the generic accounting (with a simple chart of accounts) or one that is specific for your country (if that is available).  

If you have already installed a “Fiscal Localization package” and posted transactions this option will not be available.

If you already have a chart of accounts setup, then the problem may be with the Default Accounts.

Account Receivable & Account Payable

Note that these two fields are only visible to users in the Accounting groups, so ordinary inventory users cannot view or amend them.

Creating an Odoo database

Note that if you select a country when you create the database, the appropriate “Fiscal Localization package” should be installed along with Invoicing or Accounting. If your chosen country has no localization, the generic package will be selected.