Trying to reinstall El Capitan to my MacBook Pro 2015

Follow the instructions here: Factory reset of your Mac - Apple Support.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

     If possible back up your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the target drive (will be the out-dented entry) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Install OS X and click on the Continue button.

This should install the original version of macOS that was installed at the factory. When the installation is completed the system will boot automatically. Do not proceed with the setup. Instead simply Quit and shutdown the computer.

It is a shame Apple is making this so much harder than it should be.

First try an SMC reset and a PRAM reset (hold the PRAM reset for at least three chimes so it activates several times). Then try booting into Internet Recovery Mode using Command + Option + R to try to boot to the online Catalina installer. I've found some computers will only boot to the oldest version of the macOS installer. You may have to reset the date and time after the PRAM reset.

Once you boot to the online installer whether it is Catalina or El Capitan you will need to select and erase the whole physical drive. With later versions of macOS (10.13+) you first need to click on "View" within Disk Utility and select "Show All Devices" before the physical drive appears in the left pane of Disk Utility. The physical drive should appear as something like "Apple SSD ....".

If you are booting the Catalina installer you want to erase the physical drive as GUID partition and APFS (top option).

If you are booting the El Capitan installer you want to erase the physical drive as GUID partition and MacOS Extended (Journaled).

Note, if you put an SSD on it, you will need to reformat it HFS+/GUID if you had been running High Sierra or later on it. Those force reformat the disc as APFS, which the El Capitan installer can't see directly.

Startup the computer with:

Shift-Option-⌘-R

Reinstall the macOS that came with your Mac, or the closest version still available.

Source: https://support.apple.com/en-us/HT204904