What does this mean on quickbooks 'an application error has occurred while processing your request : -90'

Thanks for reaching out to us. 

The error message you encountered, "An application error has occurred while processing your request: -90," usually appears when you're trying to update your company details without using the login credentials of the Master Administrator.

In QuickBooks Online (QBO), the master admin has complete access to all the features and functions, which is the business owner who initially set up the account. If you are the admin, you need to log in using your credentials to update the company information and resolve the application error. Otherwise, we recommend asking the admin to update the company details. Doing so will ensure the error message is resolved, and the information is updated accurately.

Furthermore, you can learn more about the different user types and levels of access in QBO by referring to this article: User Types in QuickBooks Online.

Keep in touch if you have other questions about resolving error messages in QBO. I'll be more than glad to assist. Stay safe.