Warning: The unit is not a valid weight class.

I need assistance with this error:

Warning: The unit is not a valid weight class. in ~/includes/library/lib_weight.inc.php (Line 51)

the unit is kg?

What page is giving you this error?

Checkout pages?

Then my suspicion is this comes from a shipping module. That there is an item in cart missing weight unit when calculating total weight. Or the module has an invalid weight unit configured.