Why is a png viewer not loading?

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Question 54175 is the same issue highlighted previously without solution.

This is so frustrating as it's the first time I'm using Sage and the whole process has been incredibly annoying to set up and I can't even view my plot!

I'm using SageMath 9.6, using Ubuntu on Windows.

Here's my code:

sage: vector_field(X, Y) = (0.9*X - 0.5*Y, 0.1*X + 0.8*Y)
sage: p = plot_vector_field(vector_field,
....:                       (X, -2, 2), (Y, -2, 2),
....:                       frame=False,
....:                       color="green",
....:                       plot_points=10,
....:                       axes_labels=("$X$", "$Y$"))
sage: show(p, aspect_ratio=1, figsize=5)
Launched png viewer for Graphics object consisting of 1 graphics primitive

But no png viewer is launched!