Error: Unexpected CPU variant for X86 using defaults: x86

You don’t need to fix this as it is just a warning according to source code https://android.googlesource.com/platform/art/+/master/runtime/arch/x86/instruction_set_features_x86.cc.

// Verify that variant is known.
bool known_variant = FindVariantInArray(x86_known_variants, arraysize(x86_known_variants),
                                          variant);
if (!known_variant && variant != "default") {
    LOG(WARNING) << "Unexpected CPU variant for X86 using defaults: " << variant;
}

The definition for x86_known_variants is

static constexpr const char* x86_known_variants[] = {
    "atom",
    "sandybridge",
    "silvermont",
    "kabylake",
};

If your emulators or devices are x86_64 or x86 you will see this warning which can be ignored.