You need to use
(lldb) expr phrase[i]
or equivalently
(lldb) p phrase[i]
for that
frame variable supports constant indexes (i.e. plain ol’ numbers), but if you need to use a variable or anything BUT a number, you need to use the expression command
As a caveat, the behavior of frame var vs. expression might be different in some cases when doing array-like access. This won’t affect your example (but it would if you were using an std::vector, for instance).