UoM can be very helpful - but can also be confusing.
To implement multiple UoM the 'item' should be identical and the only difference is how may items are in a 'case' or 'pack'.
But if you have an item that is packed for retail sale with multiple pieces - then that needs to be a separate 'item'. IE - you can buy small padlocks individually or in a packaged set of 4. The set of 4 would NOT be a UoM variant of the single lock - its a different product. But if each of those products is sold wholesale in a case of say 12 - you would define a UoM option 'Case12' and apply it to each item.
Ideally you define the 'base unit' to be the smallest amount you deal with - typically 'each/single/one' - and then all the UoMs are whole number multiples. This can be confusing to a manufacturer because the inventory count will be based on 'each' - although all the physical stock may be in cases of 12