Getting this error; Please use a numerically indexed array

I've created a 'for each' loop to change the format of my timestamp. However, when I run it, I receive the error message 'Please use a numerically indexed array.'

Does anyone know what I'm doing wrong?

Here you can see what is happening:

https://www.loom.com/share/3eedaf9c07b045cea31aca66ca7dcb8f?sid=822c9b89-7dc0-4895-9c82-e136b2c6197c