LA115 won't "go"

My second issue with father-in-laws mower....ok it had been sitting up for maybe a season, so since my husqvarna is needing a ring job, I thought I'd pull the Deere out and use it. Well, after I got it started (finally) it won't go! Move! Forward or reverse! I can see the fan on the pully under seat spinning, but no movement of mower.

I see the drive belt isn't tight against pulleys, just there. Sitting. Belt adjustment? Maybe end of belt I'm not seeing off pully?