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
Yes BTC show up on my wallet... I guess I can't purchases any WAVES as the decentralized exchange require 0.003 WAVES as transactions costs... Any ideas how I can fix the problem?
Try a WAVES faucet