Pre-requisite: Create a custom record type (e.g., MyCustomRecord) with some custom fields and set the Parent Records to any record type. In this example, let's set the Parent Records to Customer.
1. Edit a customer record > Custom tab > MyCustomRecord.
Notice that users cannot add or inline edit sublist line items on this custom record. Clicking the Add button displays the following error: "Please enter a value into at least one field before adding the row."
To resolve this issue, follow the steps below.
1. Navigate to Customization > Lists, Records, & Fields > Record Types > Click on the Record type to be edited (e.g., MyCustomRecord).
2. In the Fields tab, notice the custom fields are set to Show in List = No.
3. Click each field and mark the Show In List checkbox.
4. Click Save.
5. Edit the same customer record > Custom tab > MyCustomRecord and notice that the user can now add/inline edit sublist line items under this custom record.