There is a known issue in ATF that using the Set Form Value test step on condition fields errors out as ATF believes the field is read-only. A practical workaround is to skip the read-only UI validation by directly setting the value of the condition field at the server side.
1. Create a new Test
2. Create the first step of the test and choose 'Server' ==> 'Record Insert', make the step on the target table that your test works on
3. Setup the test step, ensure the condition field and its depend table field are set. Save the test step
4. Create the second step and choose 'Form' ==> 'Open an Existing Record'
5. Setup the test step and make it open the record inserted by step 3 by using the reference selecting icon. Save the test step
6. Create the third step and choose 'Form' ==> 'Set Field Values'
7. Set the rest fields you need as your original plan
8. Complete the rest test steps according your requirement