Hello I have tried to set the fixed asset account in the Expenses field in the product category so I always receive the same, however it doesn't find the GL required, it only show GL with type expenses/income to select from them.

any advise on why is this? I think before I was able to do this, but not now.  any ideas?

Regards.

The "Expense" account is actually used for several things. It is the account used on Vendor Bill line items (when recording amount you owe for the product) and the one used on Customer Invoice journal items (when recording the cost of goods of the product).

Since it is used on the Vendor Bill line as the debit account, it can also debit a fixed asset account.

You can think of it as the "Expense / COGS / Asset" account.

If you wish to purchase an Asset with a Purchase Order, you can make a product and assign the fixed asset account as the "Expense" account. Note that your Category should be Manual valuation and the product cost should be $0. The amount of the debit would come from the price on the PO.

This setup is needed so that when you receive the product you do not create a current asset (this normally happens when using Automated valuation) but instead when you record the Vendor Bill you create a fixed asset (instead of normally offsetting the amount of the current asset and moving it to Accounts Payable).

If you don't need to receive the fixed asset (only recommended if you wish to store/move it to a warehouse location) you can just skip the Purchase Order and go directly to recording the Vendor Bill. 

You don't need products at all, you can just enter no product and enter the fixed asset account on the Vendor Bill.

You can either setup the fixed asset account to automatically create the depreciation schedule, can create a schedule by using the Create Asset action from the Journal Item representing the debit to the fixed asset account, or can manually journal the depreciation. 

For more information:

https://www.odoo.com/documentation/user/14.0/accounting/payables/supplier_bills/assets.html

https://www.odoo.com/forum/help-1/storable-vs-consumable-what-is-the-difference-between-these-types-of-products-186065

A product itself is not related to an asset account - usually a account payable account, to be correctly - only the partner/vendor has a corresponding related account. But that's basic accounting 101 and nothing specific to Odoo.