-fallbackfee option in sendtoaddress I'm trying to send some bitcoins using bitcoin-cli API, but bitcoin core tells me that Fallbackfee is disabled. How can I set a value for fallbackfee? 
I've tried to specify it like
 bitcoin-cli sendtoaddress -fallbackfee=someValue but it didn't work.
 I've also set true for substractfeefromamount and false for replaceable
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.