I agree with Mark, you should put the file in your local repository instead of your cache. Especially since the expectation is that the cache could be (and often is) deleted at any time.
However, to address your question, the most likely reason for your error is that your folder hierarchy does not match the expected pattern. By default, the cache is laid out as follows:
[organisation]/[module]/[revision]/[type]s/[artifact].[ext]
So you'd have to move your file to the following directory to eliminate that error:
.ivy2cacheapache.orglucene3.0.3jarslucene.jar
Sometimes, the default pattern gets changed based on settings files so the only way to be certain of the expected pattern is to look at other jars in your cache (for example, right now, my cache has the [revision] portion at the end of the file name, otherwise, all else is the same).
However, I agree with Mark though, you really don't want to manually add things to the cache. Instead, add them to the default local repository OR better yet, create your own basic repository.
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