Will not start after sitting a few days

Hello,

This sounds like the vehicle has an excessive parasitic draw (current drain).
All electrical circuits with a battery will have some amount of parasitic draw, however due to any given faulty electrical component/circuit it can become excessive. Which will lead to the battery drain at a much faster pace.
This excessive draw can be due to a light staying on, like a glove box light, a malfunctioning switch etc.

In order to detect the draw, you will have to use a multimeter and measure the drain on the battery as you are removing different fuses for different circuits.

Here is a helpful video that shows how to do that test:

https://youtu.be/z5F7WpgjaRs

This vehicle should have less than 25 milliamps of parasitic draw.

I have attached parasitic draw test procedure below.

Please let me know if you have any questions.
Thank you.