Hello there. Thank you for reaching out to us in the QuickBooks Community forum. We understand how frustrating it can be when encountering errors while trying to add a new transaction in QuickBooks Online. No worries, I'm here to assist you and find a solution to this issue.

To better assist you, could you please provide me with the specific error message you're receiving? This information will help me understand the issue and provide you with the appropriate solution. Also, I'd like to know what "Add" button are you referring to when manually adding transactions. Any additional information such as screenshots will do. 

In the meantime, I'd like to include the complete step-by-step process in manually uploading transactions into QuickBooks Online to make sure we're not skipping a single step. 

Here's how:

1. Go to the Banking menu then select Banking.
2. Click on the tile for the account you want to upload the transactions into.
3. In the Link account ▼ dropdown, select Upload from file.
   
    
4. Select Drag and drop or select files and select the file you downloaded from your bank. Then select Continue.
5. Follow the onscreen steps to match the columns on the file with the correct fields in QuickBooks.
6. When ready, hit Done. 

Additionally, errors when performing QuickBooks tasks can be unexpected sometimes. However, you don't have to worry as we have an available reference that you can freely check out and learn what to do if you happen to encounter any: Fix bank upload errors in QuickBooks Online. 

We acknowledge that the process of uploading transactions to QuickBooks Online should ideally be seamless, and we apologize for any inconvenience this may have caused you. Please be assured that we are committed to providing you with the necessary support and assistance. If you have any further questions or concerns, please don't hesitate to reply to this message, and we will promptly respond. Take care and stay safe.