The series_sort function can be used only in template program mode or general program mode. You are using single function mode.
In TPM you would use
Code:
{:'series_sort()'}{series_index:0>2s| [|]: }{title_sort}
In GPM you would use:
Code:
program: strcat(series_sort(), finish_formatting(field('series_index'), '0>2s', ' [', ']: '), field('title'))
or possibly
Code:
program: strcat(series_sort(), finish_formatting(field('series_index'), '05.2f', ' [', ']: '), field('title'))
which better handles fractional series indices.