Does your token have capabilities to deploy to stable? It could also be a problem with the dpl v1 scripts. Try the dpl v2 scripts: https://docs.travis-ci.com/user/deployment-v2/providers/snap/
And a slightly off-topic piece of advice: Travis is probably not the CI provider to invest time into as they are beginning to shut down their “(almost) unlimited free builds for OSS repos” plans. See https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing