《牛津高阶英语词典》(2053页）这样解释：

The difference between somewhere and anywhere is the same as the difference between some and any.

由此，可以看下some和any的使用区别：

Certain determinatives, pronouns, and adverbs which, by reason of their meaning, are more usual in positive contexts are replaced by a corresponding set of non-assertive forms in negative, interrogative, and conditional contexts.Assertive forms include the some series of words(e.g.some, someone, somebody, something, somewhere) which contrast with the non-assertive any-series(e.g. any, anybody, anyone, anything, anywhere).

The demarcation between assertive and non-assertive is not rigid.

Although non-assertive forms are often impossible in positive statements, assertive forms may be possible in negative and interrogative clauses, where they suggest a markedly positive meaning.Non-assertive forms occasionally occur in positive statements.

《牛津英语用法指南》161.4节指出：在if从句中，some和any都常用。

If you want some/any help, let me know.

再如：赵振才教授也认为，some, something, someone, somebody也可以用于条件句中：

1 lf l had some money, l could buy it.

2 lf they don't like something they simply veto it.

《薄冰英语惯用法词典》例句：

lf there is some trouble, drop me a line.

所以，我认为，此题可以填somewhere，作为assertive form（断定词项），表示说话人比较肯定。但是，语意上，似乎不宜填everywhere, 比较绝对，如果改为虚拟语气，似乎可以接受，如：lf you could go everywhere, where would you go?但是，如果属于初高中考试试题，填anywhere最佳，最好不填somewhere，以免失分。

仅供参考。