Hi @sindhusomisetty ,
To make any field manadatory ,
1. Open/Edit the Metadata Schema
2. Click the desired field
3. Go to Rules and mark it as required

You can probably unlock the dafault schema via overlaying since they are locked because its a fallback schema.
Best way is to create your own schema, you can copy the default schema and add your custom Tabs/properties and then ofcourse add them to asset folders for them to reflect.
Refer - https://experienceleague.adobe.com/en/docs/experience-manager-cloud-service/content/assets/manage/me...
Regards,
Anupam Patra