You're probably looking for the Enabled property for the text box. If it's able to be modified Enabled will be true, otherwise it will be false.
A region checkpoint is probably not the best option here. Use a property checkpoint to check the text in the box after setting it.
For reference, here is the documentation for region checkpoints:
https://support.smartbear.com/testcomplete/docs/keyword-testing/reference/checkpoints/region.html?sbsearch=Region%20Checkpoint
And property checkpoints:
https://support.smartbear.com/testcomplete/docs/keyword-testing/reference/checkpoints/property.html?sbsearch=Property%20Checkpoint
The textbox already had value in there. I want to check the textbox if it can be modified or not and ignore the value in there.
You're probably looking for the Enabled property for the text box. If it's able to be modified Enabled will be true, otherwise it will be false.
