My computer is flashing between the prohibitory symbol, the question mark folder, and the apple symbol.

The way the cables in these Macs are used, they end up wearing against the machined inner surface of the case, causing shorts, and where they take tight turns, they develop cracks. Models in this era are notorious for needing to replace the cable when you replace the drive.

The litmus test is to place the new drive in an external adapter, toaster, or enclosure. It can be installed on, and can run your Mac from there. Then if it acts up when moved inside the computer, replace the cable. Some users just call it preventive maintenance and replace it whenever the drive is replaced.

The original drive from a 2012 Mac is well past its expected lifetime.

You haven't provided the version of macOS currently installed on your computer.

Basically, the question mark folder tells you that a part of the OS is missing. The prohibitory symbol means no OS could be found. So, I would reinstall macOS. It's possible that your original installation left behind a working Recovery HD. So, start by doing this:

Install El Capitan or Later from Scratch

If possible, back up your files.

1. Restart the computer. Immediately, at or before the chime, hold down the Command and R keys until the Apple logo and progress bar appear. Wait until the Utility Menu appears.
2. Select Disk Utility from the Utility Menu and click on the Continue button.
3. When Disk Utility loads select the target volume (indented entry with vol_name) from the Device list.
4. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
5. Check that the partition scheme is set to GUID.
6. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
7. Click on the Apply button, then wait for the Done button to activate and click on it.
8. Quit Disk Utility and return to the Utility Menu.
9. Select Install OS X and click on the Continue button.

The solid Apple is not in the computer's ROM. It only appears when loaded from a drive that was able to at least transfer one blob of software to memory correctly.

The question mark symbol says that a bootable drive could not be found where the parameter in Parameter RAM says it should be.

The Prohibitory symbol says that a bootable Volume was found, but on execution, one of the initial primary components was found to be damaged or wrong version or missing.

This contributor says you need to consider replacing your boot drive. ¿What year MacBook Pro 13-in? A1278 maps to 2008 to 2012 non Retina display models.

APFS is only available with Mojave and Catalina.

For most Users, MacOS Extended, Journaled (and none of the others) is the best choice.

If the best one did not work, that is most likely to indicate disk failure, not a bad choice.

How did you decide your choice did not work?

The boot drive holds MacOS, as well as all your files (unless you moved them to another drive).

When a Bad Block is encountered, MacOS uses the drive controller to re-read the bad block as many as 1,000 times, in hopes of getting good data. this can take up to a quarter minute worst case for each Bad Block.

A drive that is accumulating Bad Blocks is likely to get worse and be unusable within about 6 months time, over a very large sample of drives. It may suddenly stop working completely at any time -- you are getting your warning right now.

Users who replace the original drive with a 500-ish or larger SSD drive (and new cable) report, "It's like getting a whole new computer!"