'Ruby: selenium DriverExtensions returns unknow command

Every function from ::DriverExtensions give an erros:

POST /session/{session_id}/funciont did not match a known command

I'm specially interested in save_print_page(path):

  @driver.save_print_page('path.pdf')

but the error is the one described below:

POST /session/731e8f40-b709-4090-845d-e6bdc794fcea/print did not match a known command

can you guys help me?

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