belt tension is not adjustable, if I understand tractor still moves but
barely as belts are slipping with a new belt installed ,
1 double check that you have the right belt
2 check idler pulleys and belt route and guides
3 engine pulley is worn (has spread apart and now belt rides to far in)

those are the areas I would check.

if you have no drive what so ever I would ask if you had rear wheel off to
check or fix something, if you did make sure the key that locks the wheel to
the axle did not fall out with out that key you will go no where.