If you are making a payment from checking, you should be able to split the category and apply an amount to your liability account and another to an interest category. That will update the liability balance and add the interest to the category you chose.
That does work for me when creating a new transaction in checking.
So I'm not sure how you are being blocked from adding a split.
Hi, I just realized that I can duplicate the transaction and split it, so this is ok, but I would still like to know why that series of transactions could not be split. In trying to record the split I went through a number of prior steps that I do not recall, it may have involved a batch move from another account. Thanks