Macbook air problem

Someone please help! Im trying to sell my 2015 macbook air. I clearly made some kind of mistake when i was trying to wipe/erase my data because now everytime i start it up, it shows a file with a question mark. I followed all the steps on apples website and on other reddit forums, i ended up fixing it last night and updated it to monterey. I thought it was all ok then this morning it showed the file again??? Do i have to repeat everything now? Is it a lost cause? I dont think its a problem with the computer itself as some people mention because it worked great before i made the stupid mistake of deleting the hard drive or whatever i originally did. Please help!