Unable to Inline Edit Sublist Line Items on a Custom Record even if Allow Child Record Editing is Enabled

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.