When you add the API to amplify app, using amplify add api command, GraphQL or REST API will be add. For local environment test, you can use amplify mock api command.
However, this command is available only for GraphQL API.

When you mock the API without adding GraphQL API, the command return the error as shown below.

Failed to start API Mocking. Running cleanup tasks.
TypeError: Cannot read property 'stop' of undefined
    at APITest.stop (/snapshot/repo/build/node_modules/amplify-util-mock/lib/api/api.js:187:33)
    at APITest.start (/snapshot/repo/build/node_modules/amplify-util-mock/lib/api/api.js:150:18)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)
    at async start (/snapshot/repo/build/node_modules/amplify-util-mock/lib/api/index.js:18:5)
    at async Object.run (/snapshot/repo/build/node_modules/amplify-util-mock/lib/commands/mock/api.js:21:5)
    at async Object.executeAmplifyCommand (/snapshot/repo/build/node_modules/amplify-util-mock/lib/amplify-plugin-index.js:47:3)
    at async executePluginModuleCommand (/snapshot/repo/build/node_modules/@aws-amplify/cli-internal/lib/execution-manager.js:142:5)
    at async executeCommand (/snapshot/repo/build/node_modules/@aws-amplify/cli-internal/lib/execution-manager.js:40:9)
    at async Object.run (/snapshot/repo/build/node_modules/@aws-amplify/cli-internal/lib/index.js:117:5)

This error does not describe the actual situation. This is because the mock clean up fails when the no appsync api exists error occurs. For now, I have submitted a Pull Request to show the correct error. I’m sure it will be fixed soon.