You need to use the field key.
When a field does not already exist and have a value for an ACF field then ACF is unable to figure out how to update the field.
This is true of all ACF functions for updating values. See the section “Updating via field key” on https://www.advancedcustomfields.com/resources/update_field/
In the case of a repeater all of the sub field values you are adding to the row should also use the field key.
