It means you're not any good. Crap means ****, from a slang term for toilet, crapper, which came from Thomas Crapper, an early lavatorial pioneer.
Thomas Crapper (baptised 28 September 1836; died 27 January 1910) was a plumber who founded Thomas Crapper & Co in London. Contrary to widespread misconceptions, Crapper did not invent the flush toilet. He did, however, do much to increase the popularity of the toilet, and developed some important related inventions, such as the ballcock. He was noted for the quality of his products and received several royal warrants.
Manhole covers with Crapper's company's name on them in Westminster Abbey are now one of London's minor tourist attractions. Thomas Crapper & Co owned the world's first bath, toilet and sink showroom, in King's Road until 1966. The firm's lavatorial equipment was manufactured at premises in nearby Marlborough Road (now Draycott Avenue).