Do you have to add expenses on the computer because it won’t let me add them on the phone

Generally, no, the mobile app functions similarly to the website accessed via a computer, and they both support the same tax forms. 

To enter expenses such as mortgage interest, property taxes, etc in TurboTax Online:

1. Open or continue your return.
2. Navigate to Federal > Deductions & Credits

If you are looking to enter business expenses, here's how, from Where do I enter my self-employment business expenses, like home office, vehicle mileage, and supplies?:

1. Open or continue your return.
2. Navigate to the Schedule C section:
   1. TurboTax Online/Mobile: Go to Schedule C using search tool (magnifying glass) and "Jump to Schedule C" option in search results..
3. Answer Yes to "Did you have any self-employment income or expenses?" and answer the questions until you get the "Select any expense categories you have" screen.
   Select your expenses, then Continue.
   If you've already worked in this section, select Edit or Review next to your business.

Also, see: What's the difference between TurboTax Online and the mobile app?

BUT, I would beg you to not use a phone to prepare your taxes.

For anything but a simple W-2, the phone has such limited viewing, and your taxes are so important , that you should be using a full computer, or large screen tablet, where you can see far more of what is needed, and navigate more easily to the areas you want to deal with.   

On a phone, you just miss too much....just don't prepare on a phone. Please.