I am having requirement to show validation errors for particular feilds in the page list on a form which is already having data populated after creation of case from external API.

Eg.

Pagelist x having feilds a,b,c and data is already present on a form because case is created from external API. I want to show error message for c feild on first step form.

I tried applying edit validate for feilds advance tab but it is not working. I tried creating a step before that step to apply validation to flow action but it is showing processing error.

I tried applying validate rule to same flow action but it is not working as form is not submitted.

Basically I want to validate data which coming from external API and want to show error in pega as one engagement platform is pega.

Any suggestion...???