There can be different solution as per the configuration .You have to check all the Journals,customer,product,payment methods if Ledgers/Accounts are properly associated with it.

In my case customer 

1. Intermediary Account(Debtor (POS)/Account Receivable (POS)) was missing from POS setting.

2.In payment Method for POS 'Identify Customer' was checked but the Customer was not tagged in POS order which is mandatory for this configuration in Odoo v15.

I am also getting the same issue while I don't have accounting module. What should I do every time closing a session getting the accountable invoice line error

Go to contacts and find the customer in the account tab set account receivable and account payable on the customer. The error must be solved.

Hello, kindly help
I have tried all of the above, but unable to close the session with Error message "Missing Required account on accountable invoice Line"
Thank you

Since the error is related to the invoice line, I don't think it is because of the customer's settings, but there is product in the invoice that is not fully adjusted (you should to put an income account to each product or product category) for all products.

then try

Thanks

Please view this video for resolution. Thanks

https://youtu.be/z_3aBDOrYvI

In POS Configuration we need to set Default Intermediary Account.

https://www.youtube.com/watch?v=dIB5earK3xA&t=189s

This is the method of POS Configuration in 4 mints

i have the same issue, anyone can help?

Well in odoo15 the solution was to goto company in developer mode..
Edit it
In form view add this field in the form view

Once you have selected and refreshed the screen you would notice the field is empty. simply edit and select the relevant receivable account and save. Now you would be able to close the session accordingly and at ease.

Hello Dears,

I hope all is well. I am currently using Odoo v17 and have faced the same issue, I have followed all the recommendations and videos shared. But in the end, all it took was going through all the current session orders and making sure that they were posted. 
POST ALL ORDERS that's all

"Validation Error

Close

The operation cannot be completed: Missing required account on accountable line."