Hi atdas.
Being able to assign materials to your costs of goods sold is a great way to keep your account balance correct. I'd be happy to help point you in the right direction.
If your receiving an error message when trying to allocate material costs to your cost of goods sold it could be due to your browsers health. Sometimes your cache and cookies can get damaged or corrupted and this causes your internet browser to not show websites using the correct information. To fix this you'd need to clear your cache and cookies and try opening your QuickBooks in a private browser such as Google Chrome's Incognito mode.
If this doesn't fix the issue I recommend contacting our support team as they can check the error you're getting and the steps you used prior to having it show up. You can reach them by using one of the following methods:
Schedule a Callback or start a Chat: click (?)Help in the upper right in QuickBooks Online > type and enter "Contact support" into the QB Assistant > click Contact Us > explain your situation > click Let's Talk > choose Get a callback or Start a Chat.
Social Media: Facebook, Twitter, and Instagram.
Support hours are from Monday to Friday between 9 a.m. and 8 p.m. ET.
Have a great day!