Hello, @Constdan91.

I know how it is important to record correctly all of your transactions in QuickBooks. When invoices you send in QuickBooks Desktop become uncollectible, you need to record them as a bad debt and write them off.  

Since you already created an account called "allowance" there is no need to create an expense account to track your bad debt.

You can follow the steps above provided by my colleague except for the first steps. That ensures your accounts receivable and net income stay up to date. 

You can refer to this article for additional information. Write off bad debt in QuickBooks Desktop. 

Please let me know if you have other questions. Take care!