Cannot read property '__isVuelidateAsyncVm' of undefined

First some comments, it's great that you have provided a sandbox, but your question itself should contain the relevant code to reproduce the issue. This question becomes useless if (when) the link dies.

Doing some debugging to actually find which field is causing this error. By eliminating one field after one, I noticed that it is the gender field that is causing your issue. So looking closer at inArray, your custom validator, I noticed that you are not returning anything, which causes the validation to fail. Add return to your inArray:

const inArray = (value, vm) => {  // add return!  return vm.items.some(gender => value.indexOf(gender) !== -1);};

PS, as a hint, try to do some debugging to narrow down the issue and you might have solved this yourself :)

Rest of relevant code for your issue:

<v-select   v-model="user.gender.value"   :items="user.gender.items"   label="Gender"   :solo="true"   :error-messages="userGenderErrors"   append-icon></v-select>

Script:

validations: {  user: {    gender: {      value: {        required,        inArray      }    },  }},

FIXED SANDBOX