What is the best exchange to invest in WAVES? I only have accounts on Poloniex and Kraken and no WAVES there 
Buy from bittrex https://bittrex.com/Market/Index?MarketName=BTC-WAVESLiqui.io is also nice exchange and Bittrex is one of the best exchange. So you can buy Waves from both exchanges and can make withdraw instantly. I have good experience with them and currently using them.
Or use the waves DEX. More volume than liqui but not as good as bittrex(yet)
I sent some Bitcoin to my Waves wallet but I can't pass any orders to buy some WAVES (error). Any ideas why?
Do your BTC show up on your wallet? Or show up as wBTC