I was getting this error In my salesforce Flow where I was  using lighting web component which has some properties and we want to use data from that properties to next screen.

Solution – 

The property we are trying to use was declared as  array type [] 

@api valueToBeUseInFlow = [];

and on onClick event we have added String(event.value) to that [] variable like :

valueToBeUseInFlow = String(event.value) ;

By changing definition of valueToBeUseInFlow  array to string resolved my issue.

@api valueToBeUseInFlow = ”;