The error appears due to the Letter Type category being marked down as a Cover Letter; the 'response due in days' column for letter templates cannot have a value input and saved if that letter template is associated with a letter type category within the cover letter ticked.
To make configuration changes for this, navigate to the following area:
Click Case Management client.
Click System Manager.
Click Lookup Editor.
Click Core Case Management, Letter Type.
You can then untick the Cover Letter tick box against the Letter Type category if you so wish.
📌Note: Another alternative is to add a response due date in the case management website. When you're creating a new letter, there is a field where you can input a response due date, and below that field, it tells you how many days that would be from today's date up until the date selected. This is indeed on an individual basis.