(1)
The error you see happens because your 0.84429 reducing transaction has nothing to match against. You didn't add anything at a cost basis to reduce against. Check out the "bean-doctor context" command to help you debug what's going on, and "bean-query print".
But mainly, you should not be using the book_conversions plugin. This is ancient and if I recall it was created to answer a question on the mailing-list (search for it for context) before FIFO support was implemented. Since then, Beancount has had proper support for FIFO, LIFO booking.
(2)
Now, the other can of worms you're opening is that you're using Bitcoin. Is Bitcoin to be used as a currency or as an investment? If it's to be used as a currency, you need not care about its cost basis and such. But more likely, if you're using Bitcoin like most of the other people - to speculate - surely you want to handle it like an investment, that is, tracking cost basis. Beancount can do this, though when you use your Bitcoins to buy things, you'll have to book reductions like sales. It's not difficult, but you'll have to use syntax that hints the expense is to be reduces with a cost basis (e.g., {}). There's no great intermediate solution here.
So assuming you're happy doing that (that is, treating Bitcoins like investments) you'll have to be cognizant of writing appropriate syntax every time you add new bitcoins: supplying the cost basis when you add Bitcoins to an account, and making sure to let Beancount know reductions of Bitcoins from an account are to be carried out with the cost basis (using as little as {} should be possible, since FIFO should kick in).
I would suggest you should become intimately familiar with this doc:
http://furius.ca/beancount/doc/trading, and the FIFO and LIFO, etc. booking method bits of the language syntax documentation, and start treating all deposits and withdrawals of Bitcoins from accounts like you would shares of a company.
(3)
Thirdly, you're attempting to use both Bitcoin and Ethereum, both of which surely you'll be wanting to track cost basis for. It's not impossible, but you're making your life miserable if you're going to use this for any real world expenses. The analogy is like if you were going to buy shares of AAPL, and every time you go to the coffee shop you'll pay coffee with some fractions of shares of AAPL and always have to choose which lot to choose from every single time (yes, Beancount can figure out which lot to use, but still, you have to indicate as such). I don't think there's any system that is designed with that use case as the common case, neither Ledger, Beancount or any other, though Beancount comes the closest to having the features you need to do that.
I think if you have the time you can probably come up with a clever system of accounts where you set aside some amount of Bitcoins to a separate account stripping away the cost basis and when you spend you always book against this account. This would segregate all the cost basis tracking and trading activities to another set of accounts. I think that would be a practical and probably nicely workable compromise. I haven't tried (and cannot afford to right now) but I think there's everything there to support that use case already.