Comparing the stacktrace with https://github.com/travis-ci/dpl/blob/v1.10.15/lib/dpl/provider/releases.rb, the error happens at checking credentials.
If you are sure your token is correct, maybe it doesn’t have required permissions?
FYI, you are not using deployment v2, so refer to https://docs.travis-ci.com/user/deployment/releases/.
You can also double-check if your token is decrypted correctly by printing a small part of it.