On some occasions I have used taxis to get to work. I pay for these via an app I don’t have receipts. Can I still put them as travel expenses

It's nice to see you here in the Community forum, nav-dugmore.

In QuickBooks Self-Employed (QBSE) , you can manually add transactions to track your expenses. 

Here's what you'll need to do:

1. In the left menu, click Transactions. 
2. Select Add transaction button. 
3. Enter the necessary fields and select the expense category. 
4. Hit Save when done. 

For your visual reference about this process, you can check below screenshot:

As for your concern, if you can put them as travel expenses without having a receipt, I recommend checking with your accountant. They can guide you on what is the best thing to do in this situation. 
 

If there's anything else you need help with in managing your transactions, let me know by commenting below. I'm here for you. Have a great day!