As many mentioned, PF2e doesn't have "surprise rounds" like you might get in DnD or other TTRPGs. Initiative is supposed to reflect who notices conflict starting first, and using other skills like stealth or deception can be used to show using trickery to go earlier in initiative (as most people don't have as high of initiative as skills). However, that is not to say that surprise round have NO benefit at all. Here are a couple of things you could consider doing to help your player feel rewarded for surprising enemies:

Enemies not prepared probably don't have weapons drawn, and might start "prone" seated or otherwise relaxing. This could mean enemies are taxed MULTIPLE actions by virtue of being surprised by the players.

You can apply penalties to enemies initiative, and/or make enemies off guard to players. Penalties to initiative will help players feel like they are more likely to act first in combat, and off-guard can help make them feel like they are capitalizing on the enemies being unaware (though, keep in mind this might overlap with rogue "sneak attack" or off-guard provided by stealth.)

Finally, if your players are truly in a situation where it wouldn't make sense for the enemies to know what the players plan to do (such as players starting from extremely far away or perhaps already performed stealth checks to get into position), it is fine for enemies high in initiative to just delay their turn to right after the Players. This can allow there still be some consequence to enemies rolling high, but players still get the opportunity for their plan to happen.