Each product in Odoo linked to Product Category and you need to specify the Income/Expense accounts either in Product category or Product itself.

Go to the Inventory app -> Configuration then product categories and you have to open each category and make sure the Income/Expense accounts is set.

Income Account: This account to be used to validate the invoices of the customer.

Expense Account: This account to be to used when the vendor bill is prepared.

If you want to set Income/Expense accounts for a product to be different from his product category accounts, Open the product and in Accounting tab you can set Income/Expense accounts for the product and these accounts will override the category accounts when sell/purchase this prduct.