Help! Cannot get my Macbook to install OS after wiping hard drive

Hello,

I have a MacBook Pro Early 2015 which had OS x El Capitan 10.11.6 on it.

I had been having problems for a long time with it logging onto the WiFi (was having to turn the wifi on and off on the macbook every time I loaded a new web page!)

I had tried everything I could find on the internet to try and solve it but it was driving me mad.

So, I decided to try wiping the hard drive to make it as new.

I followed a lifewire guide on doing a clean install using the option key during startup. I managed to get it to start installing the OS but then right at the end it said "OS X could not be installed on your computer. No packages were eligible for install..." I am now stuck with it asking to restart and it doing the same thing.

I have since created a bootable USB drive containing the OS I want to install and have been following this guide https://www.lifewire.com/perform-a-clean-install-of-os-x-el-capitan-on-your-mac-2259940 but I have got to the point where I am supposed to erase the start volume and it won't let me. There seems to be various disk images that I think have been created while I was trying to get the os to work and when I try to install any of them including the one that says 'macintosh HD' it says 'erase process has failed - couldn't unmount disk'.

Having googled that error there are various suggestion on how to deal with it including deleting things through the terminal but I am just not tech savvy or confident enough to do this. I am worried I will make things worse (if they could get any worse!)

If anyone can advise that would be great otherwise I might have to pay someone to look at it which I can't really afford right now!

Thanks in advance!