There are a number of reasons why you may not be eligible for an instalment plan on your credit card, such as:
• The transaction you're trying to add is not valid (see here for what an eligible transaction is)
• You may have a promotional rate on your account, meaning you pay no interest or fees on certain transactions until your rate expires. (Since an instalment plan comes with a fee, this option would be a more expensive way for you to repay your purchase)
• You may have withdrawn cash at an ATM or used your card to get some travel money
• You may not have enough available credit on your card to cover the transaction amount plus the instalment plan fee
If you don’t see the option to set up an instalment plan, keep checking back as you may be eligible in the future.