Early 2009 Macbook won't install El Capitan

Interesting issue or failure. Try using another brand of USB stick or use a USB hard drive/SSD for the installer. An SMC reset & PRAM never hurt if you haven't tried them yet, although it is a long shot.

Before you get rid of it, why not try installing Linux on it? Believe it or not I've recently encountered a couple of Macs where macOS would not work at all due to some unknown hardware issue, but Linux would run perfectly.

One of the Ubuntu flavors such as Kubuntu or Ubuntu-Mate would be nice or Linux Mint. I personally use Debian (the source of the others I listed) as it seems to run better and is more stable, but it requires a bit of extra work to identify & install some non-free drivers to get it fully functional. The three distributions I mentioned should be easy to install and I believe will just work out of the box on an older Mac especially if you have 4GB of RAM (3GB may be Ok, 2GB is very tight). The default Ubuntu with Gnome desktop needs a very powerful system which is why I didn't suggest Ubuntu proper, but the others I have listed should work fine from what I've been told. All of the Ubuntu flavors use the same underlying core components, but change up the GUI interface (aka Desktop Environment) and the default apps installed.

I would suspect you may have a failing or corrupt SSD. How long was the computer left without power? I've seen a few reports that SSDs can become corrupted or get into an odd state after just six months without power. Most sources suggest it usually takes a year or two or three without power for an SSD to become "corrupted".

If it was my computer I would perform a hardware ATA Secure Erase on the SSD to reset it to factory defaults. This should correct any "corruption". You can perform the ATA Secure Erase using Parted Magic which is a customizes Linux utilities boot disk. Create a bootable Parted Magic USB drive using Etcher (Mac/Windows/Linux).

Option Boot the USB drive and select the orange icon labeled "EFI". On the Parted Magic desktop there should be an icon for erasing drives. Within the app you need to select the "ATA Secure Erase" option which will require you to sleep & wake the laptop and to provide a temporary password to activate the hardware secure erase process (keep the password simple). The hardware erase usually only takes about 30 seconds to several minutes to complete.

The last free version of Parted Magic is available from the MajorGeek's link here.

This page describes the hardware secure erase process using Parted Magic although it shows it using a newer version of Parted Magic. The interface of the older free version is a bit different. Only the ATA Secure Erase will reset the SSD to factory defaults.

Did you first erase the SSD as GUID partition and MacOS Extended (Journaled)?

Did you just download the new El Capitan installer recently or is it one you've had for a while?

Is your date & time correct on the MB?

How far into the install process does it get before you get this informative error message?

An old computer is the perfect way to try it out since you have nothing to lose. You can easily download and install other common third part browsers such as Chrome and Vivaldi (and I believe Brave as well) if they are not included in the repositories.

I'm not sure what documentation Ubuntu and Linux Mint provides, but if you find it lacking, then check out the Debian Administrator's Handbook. While a bit old now most of what it contains is still valid and the majority of it still pertains to Ubuntu & Linux Mint as well, although both of these distributions have slightly different styles of repositories for their software.

The Arch Linux Wiki pages are also a great source of information even though the Arch distribution handles software repositories and software installs differently. Most of the Arch Linux configuration information can easily be adjusted to work with any of the Debian & Ubuntu derivatives.

If you find the Trackpad configuration to be lacking, then you may need to install the older "xorg-driver-synaptics" or "xserver-xorg-input-synaptics" driver which allows for more configuration options (especially for sensitivity and scrolling options) than the recently revamped driver.

If you have any issues or questions, then just update this thread.