https://github.com/softonic/axios-retry/pull/303/files

Undefined headers make axios-mock-adapter incompatible with axios-retry because it assumes that error.response.headers will not be undefined if error.response is present.

I'm trying to get this fixed here too, but opening this to track the bug incase anyone else runs into it.

TypeError: Cannot read properties of undefined (reading 'retry-after')
    at retryAfter (node_modules/axios-retry/dist/cjs/index.js:67:145)
    at exponentialDelay (node_modules/axios-retry/dist/cjs/index.js:85:45)