[TYPO3-Solr] Index Queue Worker (solr) Failures

[TSniper]

I have deleted all tasks on scheduler and i reinstall the solr extension, after that i create a new task on scheduler and i got this error :
Class tx_solr_scheduler_IndexQueueWorkerTask of the registered task could not be found. You should re-install the extension that provided it or simply delete this task.

But what does that mean, the solr is already installed now, why i have this error ?