I want to ensure this is taken care of, @marcia13.
Usually, the possible reason QuickBooks keeps on switching to single-user mode is because of the network setup. We can download and run the QuickBooks File Doctor. This will fix company files and network issues in QuickBooks Desktop. For the detailed steps, please check this article: QuickBooks File Doctor.
Also, we make sure that your Database Server Manager is properly set up. This is the tool that allows users to access the company data. Please check this article about QuickBooks Database Server Manager and let's perform all the steps provided.