An application may stop functioning properly due to various reasons such as being outdated version, excessive consumption of system resources which hampers smooth operation, and corrupt files resulting from incomplete updates. I have some details on what you can do in resolving the issue, ZRP.
QuickBooks Self-Employed and QuickBooks for Solopreneur are often referred to as QuickBooks Online for Self-Employed or for Freelancers and cater to individuals operating their own businesses. Can you confirm which of the (2) two products are you using? You can do this by logging in to your account using a web browser. Once logged in, the dashboard will usually indicate what product you use. I've added a screenshot for you to check:

If you're using QuickBooks Self-Employed (QBSE) account, you can contact our support team via the Contact Us option within the app to check it there are issues. You can also choose this link: What can we help you with? and choose QuickBooks Solopreneur to be connected with our QBSE US live support.
But if you're using QuickBooks Solopreneur, you'll have to make sure that you're using the application dedicated for the product. You can download from this link the iOS or Android mobile app, and then install on your phone or tablet, and sign into your account.
However, if you're still having the same problem, I recommend reaching out to our support team to investigate the source of the error. Please make sure to follow the steps outlined under the Solopreneur product. See the screenshot below for guidance:

If you'd like to add transactions manually in QuickBooks Self-Employed (QBSE), take a look at this helpful article: Manually Add Transactions in QuickBooks Self-Employed.
Additionally, learn how QuickBooks puts your transactions accurately for your Schedule C. I've got this resource for more details: Categorize Transactions in QuickBooks Self-Employed.
Post your reply below if you have any other questions or concerns when adding an expense transactions. I'm always right here ready to assist you further.