As of now, Zoho CRM does not support validation rule on lookup field.
Though, you can create validation rules for other fields, by following the steps given below:
- Tap on Setup, Customization and then choose Modules and Fields
- Choose a module for the validation rule
- Tap on the Validation Rule tab
- Press the ‘+’ button
- Choose the field that you wish to validate
- Tap Next, and then configure the function
- Save changes, and the changes would be reflected.