unable to reinstall OS X (El Capitan).

All things being equal, the SSD is the weakest link assuming there is no accidental liquid damage.

If you feel it is worth spending money on this laptop, you can replace the original Apple SSD with an OWC Aura SSD which is a drop in replacement.

https://eshop.macsales.com/shop/ssd/owc/macbook-pro-retina-display/2013-2014-2015

Edit: With any third party internal SSD you will be restricted to using macOS 10.13+ since earlier versions of macOS don't have the necessary NVMe driver for the third party NVMe SSD. This means you will need to boot into Internet Recovery Mode using Command + Option + R to attempt to boot into the online macOS 12.x Monterey installer. Unfortunately some Macs may not boot to the latest compatible online installer.....if that happens, then you would need access to another working Mac that is compatible with macOS 10.13 to 12.x in order to create a bootable macOS USB installer. I would recommend testing whether you can access the online macOS Monterey installer before making any purchase unless you have access to another compatible Mac.

It sounds like the internal boot drive has most likely failed, but it really helps to have some more important details such as the exact model of the Mac and version of macOS that you had been using on it. And does it have the original Apple OEM drive or has it been replaced with a third party drive?

You can get the exact model of the Mac by entering the system serial number on the check coverage page (please don't post your serial# on the forum since it is considered personal information):

Check Your Service and Support Coverage - Apple Support

You can also try running the Apple Diagnostics to see if any hardware issues are detected.

The SSD is where the data is stored and it appears the SSD is not communicating with the Logic Board.

If you don't have a backup, then you will need to contact a professional data recovery service to see if it is possible to recover any data from it. Recovering data from a bad SSD is difficult to impossible. If you used iCloud, then you can still access those files, but keep in mind that iCloud is not a backup option for macOS.

FYI, in the future make sure to always have frequent and regular backups of the computer and all external media (including the cloud) which contains important & unique data. There are lot more new ways to permanently lose access data stored on the internal SSD of the more recent Macs due to all the hardware, software, and security changes. Apple provides the Time Machine app to make it easy for people to back up their Macs.

You can remove the internal SSD and physically destroy it. For a 2015 model it is easy to do and only requires a PentaLobe 5 and a Torx T5 driver in order to remove the bottom case and the SSD respectively. Just make sure to disconnect the battery before removing the SSD. And make sure to screw the SSD screw back in so the next owner can install a third party SSD.

You can review the guides on the iFixIt website or the video guide for the SSD upgrade product page from the OWC (aka MacSales) link I provided in an earlier post.

Just make sure to also remove this laptop from your AppleID & FindMy and make sure if a firmware password lock was enabled, that you disable it before selling the laptop.