Validated inventory adjustments create 'done' stock.move.line records, which cannot be deleted, hence the source of those stock.move.line records (the adjustment) cannot be deleted. For traceability purposes and to ensure that Odoo can properly account for what stock and product moves resulted in you having the current QoH, the way you would correct your inventory adjustment is via a new inventory adjustment. 

Also, you can export your inventory adjustment and simply reverse everything to save time.

The best thing is to reverse the inventory you just validated because Odoo is meant to keep the stock.move.line record intact and any way to remove it, would may cause you a problem with other parts of your system.

What I suggest is either you go to your old inventory. Export it and reverse the quantities then import it again, or id the inventory is small you can just go though each product template and adjust old quantity from the quantity on hand tab.

Deleting a validated inventory adjustment in Odoo is a bit tricky cuz it's locked down for data integrity. But hey, there're ways to work with it. First, check you got the right permissions to mess with inventory adjustments. If not, ping your admin.

Now, if you're lookin to remove it, you've gotta reverse any moves tied to the adjustment. Go to the inventory module, find your inventory adjustment, and see if there's a "Reverse" button poppin up. If it’s there, hit it, and it'll make a draft stock move to cancel out the validated one. After reverse, you can cancel the adjustment.

If that button’s missin, you may hafta create an opposite adjustment manually to fix any discrepancies. It's kinda cumbersome but keeps your inventory neat. Oh, before ya start, back up anything important, to dodge irreversible goofs.

And btw, if you're still stumped or can't find certain options, check the community forums or the odoo docs for more details specific to your version. Yeah, Odoo's logic here can be weird sometimes.