There is too little information about the problem. Are there any existing signed rpms? Is there something listed in koji log? Like any errors, etc.

Here are few signed rpms. There is no error as such for this command, but it is giving empty.
(rmandadi-koji-py3) [rmandadi@brew-qe-master-2 kojitest]$ rpm --checksig *.rpm
2ping-4.1-5.fc30.noarch.rpm: digests SIGNATURES NOT OK
389-admin-console-1.1.12-8.fc30.noarch.rpm: digests SIGNATURES NOT OK
389-console-1.1.19-4.fc30.noarch.rpm: digests SIGNATURES NOT OK
4th-devel-3.62.5-3.fc30.i686.rpm: digests SIGNATURES NOT OK
4th-devel-3.62.5-3.fc30.x86_64.rpm: digests SIGNATURES NOT OK
fedora-gpg-keys-30-1.noarch.rpm: digests SIGNATURES NOT OK
flash-player-npapi-32.0.0.207-release.x86_64.rpm: digests OK
gnome-shell-extension-netspeed-3.30-0.3.20190430gita62e19b.fc30.src.rpm: digests OK
json-c-devel-0.13.1-3.fc29.i686.rpm: digests signatures OK
maven-3.5.4-4.fc29.noarch.rpm: digests signatures OK
maven-3.5.4-4.fc29.src.rpm: digests signatures OK
ninvaders-0.1.1-alt2_16.x86_64.rpm: digests OK
rdate-1.0-6.src.rpm: digests OK
redhat-internal-cert-install-0.1-9.el7.csb.noarch (1).rpm: digests SIGNATURES NOT OK
redhat-internal-NetworkManager-openvpn-profiles-0.1-30.el7.csb.noarch.rpm: digests SIGNATURES NOT OK
redhat-internal-NetworkManager-openvpn-profiles-non-gnome-0.1-30.el7.csb.noarch.rpm: digests SIGNATURES NOT OK
subscription-manager-1.24.13-1.el7.src.rpm: digests signatures OK
test-1.24.13-1.el7.src.rpm: digests SIGNATURES NOT OK
wget-1.5.3-10.src.rpm: digests OK
xchat-1.4.2-6.src.rpm: digests OK
xmms-uade-2.13-11.fc29.x86_64(1).rpm: digests SIGNATURES NOT OK
xmms-uade-2.13-11.fc29.x86_64.rpm: digests SIGNATURES NOT OK
xtoolwait-1.2-5.src.rpm: digests OK
yank-0.8.3-7.fc30.src.rpm: digests SIGNATURES NOT OK
yubikey-ksm-1.5-16.fc29.src.rpm: digests SIGNATURES NOT OK
yubikey-manager-0.6.0-4.fc29.noarch.rpm: digests signatures OK
yum-3.4.3-518.fc29.noarch.rpm:yum-cron-3.4.3-518.fc29.noarch.rpm: digests signatures OK
yum-metadata-parser-1.1.4-22.fc29.x86_64.rpm: digests signatures OK
zeal-0.6.0-1.fc29.x86_64.rpm: digests signatures OK
(rmandadi-koji-py3) [rmandadi@brew-qe-master-2 kojitest]$

We could improve the help, but this behaviour is expected. You need to pass at least one filter. I'll leave it open to enhance help message.

Ok, here i am seeing the following with options.
koji list-signed --rpm=yubikey-ksm-1.5-16.fc29
2019-08-27 14:35:11,149 [ERROR] koji: TypeError: 'NoneType' object has no attribute 'getitem'
and
koji list-signed --tag=f29-build-zauw
getting tag listing
got tag listing

PR #1631

Commit d3ef0068 fixes this issue

Commit b1db812e fixes this issue

Instead of creating new issue, writing the issue here, As per API call description, gave options debug, and key like this.

koji list-signed --key=429476b4
Usage: koji list-signed [options]
(Specify the --help global option for a list of other help options)

koji list-signed --debug
Usage: koji list-signed [options]
(Specify the --help global option for a list of other help options)
koji: error: At least one from --build, --rpm, --tag needs to be specified.

Other options are working with extra option --debug.
koji list-signed --key=429476b4 --rpm=rootfiles-8.1-23.fc29.noarch.rpm --debug
No copy: /mnt/koji/packages/rootfiles/8.1/23.fc29/data/signed/429476b4/noarch/rootfiles-8.1-23.fc29.noarch.rpm

koji list-signed --key=429476b4
Usage: koji list-signed [options]
(Specify the --help global option for a list of other help options)
koji: error: At least one from --build, --rpm, --tag needs to be specified.

Ok, that works as expected:
1) here is missing any filter (build, rpm, tag)
2) dtto
3) This one is probably also ok (it means, that you've no access to /mnt/koji/... path, which is probably right in your env).

This issue has been migrated to Fedora Forge:
https://forge.fedoraproject.org/koji/koji/issues/1552

Please continue any further discussion there.