What do you mean by “cost invoices?” There are no such things in Manager.
What do you mean by “assign it to the fixed cost” or “assigned to fixed costs?” Again, there are no such things in Manager as fixed costs.
Fixed assets are not fixed costs. They are capitalized assets that are depreciated over time. Based on your description, you are converting an inventory item to a fixed asset. For that, you need a journal entry. The way to do this was already explained to you in another thread a few days ago. You marked that explanation as a solution to your issue. What has changed? Why are you asking again?
As for your locked financial records, you can simply remove the lock date in Manager, update your records, and reset the lock date. Your problem with having sent financial statements to authorities you will have to solve on your own. The fact is, you made an accounting mistake by not recording the conversion when it occurred. Surely there is a method for submission of corrected statements.