I was seeing the error message in this question’s title and this solution worked for me:
Add Vue.config.devtools = true to the file where you create the Vue instance (main.js for me).
Note that, as mentioned in this answer, you need to put the Vue.config.devtools = true line before you create your store in order for the Vuex part of the devtools to work. If you’re creating your Vuex store in a separate file (e.g. store.js), you may need to have the Vue.config.devtools = true line in both your main.js file as well as the store.js file.
Below is what the changes looked like in my project:
