If you want to apply conditional validation on 'Practical Exam Marks' or 'Passing Marks', then Obtained Marks must be less than '35'
{{userForm.controls?.obtainedMarks?.errors?.lessThan.message}}
If you want to apply conditional validation on 'Passing Marks' then you need to add 'Obtained Marks' field less than '35'.
Conditional Expression with type 'string'
{{userForm.controls?.passingMarks?.errors?.lessThan.message}}