Cannot reinstall OS after erasing HD on old MacPro

I have a late 2011 Mac Pro that I don't use anymore. I decided to sell it so followed instructions to erase HD on Utility Disk in Recovery Mode. It erased it successfully but when I instruct it to reinstall OS, I continue to get a message that laptop is unable to connect to server. My WiFi is connected. I googled this and found that Apple has made it difficult, if not impossible, to reinstall High Sierra OS and the particular server has been down for years. Am I screwed in not being able to reinstalling the OS and now cannot sell it? I wish I had known this before as I had done the erasing and reinstalling previously after I got a replacement laptop in 2017 but thought I would do one more time before putting it up for sale. What are my options?