You can submit two types of expense:

Receipt: receipt for something you paid for that needs to be reimbursed. Expenses are generic and don't have the details of the person who paid.

Invoice: bill from the vendor directly addressed to an admin of the collective. They must be addressed to the collective at the attention of one of its members.
Please check all the requirements for expenses here!
Quick checklist:

The invoice is not addressed to A4C
I am not using my own account to pay back someone else
I did not upload a bank statement or quote request
The invoice has a tax mention
The invoice mentions all data of the supplier
I did not upload an order confirmation
I uploaded the invoice in the currency mentioned on it (and used the built-in converter)