You might need to use probes from a fleet aimed towards it.

In other words, you have to be able to actually see the system and it's planets, not just the expanding circle, general location given by a quest on the galaxy map.

You need to see the system and you're probably not close enough with your empire.

The solution is to launch probes from your fleets towards that circle and they should reveal the system.

Try that and then you should be able to send your fleet to the system.

Note: I didn't load your save. I am guessing this is the problem because I had something similar happen in one of my games.

The general circle-animated location isn't enough to plot a course. I had to use probes.

This must be a quest bug. I saw this reported somewhere else here too. Same issue. They couldn't reach a system even with the right tech researched.

I can send a fleet outside a starlane to another unlinked system.

But - you are right. I can't send it to the system you need in your game.

I loaded the save and no way can I do it.. But yet, the other fleet went happily on its way to where I sent it with no starlane link.

I can go to Zosmonos via wormhole but not to Serte.

BUG for sure..