Hello, and welcome!
So, let's see if I'm understanding you correctly. You have two checkboxes and if either one of them is selected, you require text in the textbox.
If that is correct, the condtions for data validation should be (based on your checkboxes being true if checked, false if not -- you may need to adjust these conditions for your scenario):
TextBox is blank and Checkbox1 is true
or
TextBox is blank and Checkbox2 is true
I'm attaching the sample I used to verify this. Right-click the link, save as and open in design mode. Then you can preview it to test the validation on the text field.