Why is my film coming out so dark?

It appears you are simply under exposed a bit.

If you load your photo into a histogram program such as https://pinetools.com/image-histogram [Or in an image editor] and look at the individual channels you will see that the red and green channels are far over to the left side, while the blue channel has an arc that spans the whole width.

If this is the root of the problem then it is caused by your light meter assuming it is being pointed at something that is 'middle grey', but you're pointing it at something much brighter and not telling the camera to do any compensation.

You will need to look up the exposure compensation controls for your model of camera, and learn to apply them to help your meter make sense of what you're pointing it at when working in scenes outside a 'normal' lighting range.