Alexandre Lacazette wants a longer contract at Arsenal than the club have been willing to offer so far, according to David Ornstein.

David Ornstein reports for The Athletic that Alexandre Lacazette is on course to depart from Arsenal as a free agent, with little success in contract negotiation talks so far.
The problem is that Lacazette wants a longer duration for his next contract than Arsenal have been willing to offer, and there’s little sign of a compromise at this point.
L’Equipe (via Mirror) recently reported that Arsenal were offering a one-year extension to Lacazette’s deal, which expires this summer.
You can understand why Lacazette might want some more security than that, given he’s now 30 years old.
If he signs on, we’ll be having all these same conversations about his future in seven or eight months.

At the same time, things are going well for Lacazette at this moment in time.
He’s won a starting spot thanks to Pierre-Emerick Aubameyang’s off-field issues, and the team have been better off for it over the last month.
Arsenal are finally heading in a positive direction and the Frenchman is part of that picture. So in a way, it would be a strange time to move on.
There isn’t a senior striker at Arsenal without some uncertainty about their future right now, with Aubameyang sidelined, Eddie Nketiah and Lacazette running down their deals, and Folarin Balogun heading out on loan.
There’s certainly a lot to sort out in the position this summer.