2004 MTD model 609 shift problem

I have a MTD model 609 from Tractor supply that was slipping while in gear, belts were old and wore out/ Put new correct number belts on upper and lower transmission drive. Now when in gear the only way I can get it to stop or shift is to turn off the key killing the engine. The lower belt has play in it in neutral the upper belt is tight. Runs fine whem]n in gear just can't shift while running. Need help

In order to shift both belts need to run a litle slack.
When the clutch/brake is pushed in the tensioning pulley should take the tension off the belts.
Get under there and have some one work the brake and the incorrect belt routing will jusm out & bite you.
When it was apart you should have put a little lube on the sliding sheave