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Find all the files in /etc that have .config and starts with letter a to the files that starts with letter k, that's means all the files from a to k in /etc .config.

Those filenames would match the filename globbing pattern [a-k]*.config ("starts with any of the letters a through to k, and ends with the filename suffix .config").

If you're only interested in files located in /etc, then the following command would list these files:

printf '%sn' /etc/[a-k]*.config

That command would also list directories and symbolic links (etc.) that matched the pattern. To restrict the list to only regular files (and symbolic links to regular files), then use a loop:

for pathname in /etc/[a-k]*.config; do
 [ ! -f "$pathname" ] && continue
 printf '%sn' "$pathname"
done

If you are interested in looking into subdirectories of /etc and to restrict the search to only include regular files, then use find:

find /etc -type f -name '[a-k]*.config'

This would look in and below /etc for regular files (-type f) that has filenames matching the given pattern.

The result of the commands above would be a newline-delimited list of pathnames of files. This list should probably not be used as an intermediate list of pathnames if you're planning on doing further processing of the files. Instead, use the loop above to process the files individually, or use find with its -exec option to process them. This is because the delimiter used in the output, newline, is also a valid character in Unix filenames (granted, you very rarely see it in use, but none the less).