You must set a day larger than 0 to get the validation pass. However, you can use a new prescription code "--" to mention the day is not applicable.
Steps:
1. Admin -> Code & Abbreviation- > Prescription Code -> New a prescription code

2. Type "--" and 0 in code and amount.

3. Choose duration as the type

4. Check Is Public if needed
5. Press Create

6. In consultation, you can select "--" in Days field for the drug and pass the validation

7. If you print out the drug label, "--" will be placed in the duration field. For example, Day(s): --