Hi,

Please refer to the links:

1. https://www.odoo.com/documentation/17.0/applications/inventory_and_mrp/purchase/products/uom.html

2. https://www.youtube.com/watch?v=VN1DZaF3sUI

3. https://www.youtube.com/watch?v=NQKRLB6nY-I

Hope it helps.

i think kaushik is right, see here

i know i did that

only thing that changes is that this option comes op on the purchase tab

Purchase Unit?  Units

But i cannot change the purchase unit. it is stuck 

You need to enable the Units of Measure option from Product Catalog in Sales section in General Settings.