I see no reasonable scenario in which the United States could lose, though there are many PODs that could result in them doing considerably more poorly. The war certainly could have lasted longer and American casualties could have been far heavier.
Well, you can have one with a POD that involves the government investing less in the 1880s naval expansion. Perhaps you can butterfly away that incident in which a Spanish ironclad visiting New York caused a panic because there was heightened tension with Spain, and it was realized that that ship could decimate the US Navy. But don't take it too far: without an ironclad fleet of significant size, the US isn't going to challenge the Spanish.
You can have the government invest a lot less in the 1880s naval expansion. The US Navy can have much lower training and equipment standards than it did in OTL, and perhaps you have a lesser fleet (not too small though, or there will be no declaration of war). Say the US Navy is the sixth or seventh largest in the world, rather than the fifth.
With a significant (but smaller) fleet, no real concern over its capabilities (the US government apparently didn't know the army was woefully unprepared to fight a campaign in Cuba), the US Navy could lose to the Spanish Navy. Either the Spanish immediately act to disrupt the invasion and decisively engage the US fleet, or the events go like they did at first in OTL: they remain blockaded in port for a while, and then try to break out. In OTL, the Spanish were annihilated with minimal US losses. Here, the Spanish could win.
Perhaps the victory gives time for Spain to send naval reinforcements if it sees it fit to do so.
With Spain now ruling the seas around Cuba, it becomes impossible to land or support an invasion force.
Meanwhile, the US Navy will also likely give a poor performance when it reaches the Philippines and fights the Battle of Manila Bay. In addition, there is the possible scenario that the Spanish will have mined the bay (Dewey ordered his ships forward despite the risk of mines, but there were none).
Overall, the US fails to capture Cuba and the Philippines. Spain holds its own, but it knows that it was lucky, and it isn't likely to try anything stupid like attacking the US mainland. All its assets will be engaged in holding back the US. No harsh peace terms are imposed. Spain wins a limited victory, and the US suffers a military setback, but not overly devastating defeat.
That's the best I could do.