Setting up my crowdsale rate

I need help urgently with setting up my rate. I want to make 1ETH = 0.1TKN.

But I can't seem to use 0.1 as the rate, I keep getting Error encoding arguments: Error: invalid BigNumber string (argument="value", value="0.5", code=INVALID_ARGUMENT, version=bignumber/5.5.0).

You can't set decimals number on chain. Please show us how you are trying to set the rate