C : Value escapes local scope?

So I have the following toString function:

/*
 * Function: toString
 * Description: traduces transaction to a readable format
 * Returns: string representing transaction
 */

 char* toString(Transaction* transaction){
    char transactStr[70];

    char id[10];
    itoa(transaction -> idTransaction,id, 10);
    strcat(transactStr, id);

    strcat(transactStr, "\t");

    char date[15];
    strftime(date,14,"%d/%m/%Y %H:%M:%S",transaction -> date);
    strcat(transactStr, date);

    strcat(transactStr, "\t");

    char amount[10];
    sprintf(amount,"%g",transaction -> amount);
    strcat(transactStr,"$ ");
    strcat(transactStr, amount);

    return transactStr;
}

CLion highlights the return line with a warning: Value escapes local scope (referring to transactStr)

I need to know why this is happening (I'm new to C, btw)