Resetting my MacBook Pro to Factory Settings

I need help with my MacBook Pro. I am selling it, however when I've tried to reset the MacBook to factory settings, I am getting a "timed out" message.

The long story is, I stupidly decided to download the beta version of Catalina to my perfectly operating MacBook. This was the biggest mistake ever as I've had nothing but problems and it sounds like I'm not alone in this.

I've decided to upgrade to a bigger capacity MacBook, so I'm going to try and reset the one I have now and sell it on, so I've Googled instructions on how to reset it back to factory settings. I've signed out of Messages, Apple Music and iCloud. I restarted the MacBook, held down Cmd + R and selected disk utility. Selected my Hard Drive and pressed "erase" but all I get after a couple of seconds is a message saying "The operation couldn't be completed. timed out."

Can someone please give me advice with as little jargon as possible!!