Can you nest columns/column in a gutenberg custom template?

I believe the cause of the problem has nothing to do with columns and nesting columns within columns,.

The errors are related to the very first block added in the template and a mixup of the syntax used in the examples on wp.org when reproduced.

If we clear up the indentation you’ll see that this is what you did:

[
    [
        'blockname',
        {options}
    ],
    [ child blocks..]
]

But .org does this:

[
    [
        'blockname',
        {options},
        [ child blocks..]
    ],
]

To make it clearer here is the current example on .org:

const TEMPLATE = [ [ 'core/columns', {}, [
    [ 'core/column', {}, [
        [ 'core/image' ],
    ] ],
    [ 'core/column', {}, [
        [ 'core/paragraph', { placeholder: 'Enter side content...' } ],
    ] ],
] ] ];

And if we expand and fix the indentation:

const TEMPLATE = [
    [
        'core/columns',
        {},
        [
            [
                'core/column',
                {},
                [
                    [ 'core/image' ],
                ]
            ],
            [
                'core/column',
                {},
                [
                    [
                        'core/paragraph',
                        { placeholder: 'Enter side content...' }
                    ],
                ]
            ],
        ]
    ]
]

I expect the indentation is non-standard to make it appear shorter/friendlier, but can be misleading if you don’t pay careful attention.