You've got me here to help ensure that you'll be able to post bank deposits in the QuickBooks Online app.
Unexpected behavior while working with QuickBooks Online can be caused by the stored data on the app. This can be the reason you're unable to post a bank deposit.
To fix this, let's refresh your QBO app by clicking Help, and then Reset App Data.
Next, clear the cache by going to File in the upper left corner, and then Clear Cache. This step will help us clear out the stored files which impacts and causes issues in the program.
If refreshing the app doesn't work, try uninstalling and reinstalling the app to resolve unexpected behavior, and ensure updated security settings are downloaded.
Reinstall the Desktop app through this link: http://quickbooks.intuit.com/apps/. Below the Get the QuickBooks Mac or Windows App section and click Free Download.
In the Choose an account field, select the bank account to which to deposit funds.
Enter the Date.
In the Add funds to this deposit table, enter information about any additional items to include in this deposit, including who the item was Received From, the Account, Payment Method, and Amount of the item.
Click Save and close.
You may also check out this article for more information about this procedure and for future reference: How to record bank deposits
Let me know how this goes. And if you have other concerns or QuickBooks questions, just let me know. I want to ensure your success. Always take care!