Can you still file a claim on insurance policy when the death occurred in 1998?

Sure, if the policy is still in force and a claim was never made on the policy before. It is surprising that no one had filed the claim before. Just because you found an old life insurance policy in a box doesn't mean it wasn't claim previously. The executor of the estate would have had the obligation to search records and files and make claims on any active life insurance policies. Even if they didn't have the policy, they could have signed a proof of lost policy form and filed the claim on the policy after death. It's worth checking with the company for to make sure. If it is still active they will pay interest in addition to the death benefits.