Failed to pull singularity image - no response received from singularity hub

[cahlstrom]

Hello,
I'm receiving an error when attempting to run on existing Kraken2 reports. My script is below, followed by the error message. Do you have any suggestions?

Thank you!
Christina

snakemake -s Snakefile_downstream_only --configfile /pathtofile/kraken_output.yaml -j1 --use-singularity --cores 8

Pulling singularity image shub://bhattlab/kraken2_classification:kraken2_processing.
WorkflowError:
Failed to pull singularity image from shub://bhattlab/kraken2_classification:kraken2_processing:
FATAL: While pulling shub image: failed to get manifest for: shub://bhattlab/kraken2_classification:kraken2_processing: no response received from singularity hub