If you search the archives, you'll see thata this error occurs because SPM invokes a very weak level of significance during estimation, using a kind of omnibus F-test that looks for any effect. If no voxels meet this threshold, then it prints the error you found.
Presumably the problem is that you _do_ have significant voxels ("significant," meaning meeting this rather liberal, generic F-test), but none of them lie withinyour explicit mask.
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