In product main form (information tab) you will have default UOM (Unit of Measure).  In procurements tab of product, Your purchase UOM must belongs to same category of default UOM's category.  

Eg: Your Default UOM in main (information tab) form is "grams".  So its category is "Weight".  Now, what ever you give the UOM in procurements tab of a product must belongs to same "Weight" category.  Some thing like 'grams' or 'kilo grams'.  

Hope this helps.

I'm getting exactly this message, but I have the same UOM on procurements and also on the main form.

I think is something about the access rules, but I already check and Sales/Manager have already read/create/etc options allowed on product template.

Where is comming from this problem?