-fallbackfee option in sendtoaddress
It seem like you tried to run fallbackfee in the terminal ? fallbackfee is a configuration option and should be put inside a bitcoin.conf file, if you dont have this file on your bitcoin directory you can create it: https://en.bitcoin.it/wiki/Running_Bitcoin#Bitcoin.conf_Configuration_File

Then add your fallbackfee=someValue (without the -) inside your bitcoin.conf file on a new line then restart your bitcoin client and try your command again.