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This is a problem with an old version of vue-test-utils. findIndex is not supported in IE, so we have removed findIndex from recent versions.
If you cannot update to the latest @vue/test-utils, you can add a findIndex polyfill before you run the tests—https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/findIndex#Polyfill.
Source:stackexchange.com