The Approval Action that is available by default as part of the Approval Request object, contains references to default Salesforce and Apttus profiles which your organization may not use. If you decide to create a custom object to use instead of the default Approval Action, you must ensure you handle $User.id correctly when creating the formula.

$User.id is limited to 15 characters, while some of the fields you compare it to may contain 18 characters. To accommodate this, you must ensure you concatenate any fields that are compared to $User.id, such as in the following example:

(OwnerId=$User.Id Left(Apttus_Approval__Assigned_To_Id_c,15)=$User.Id)