Calculate fees with Blocktrail/btc.com SDK
I am not sure as have not used their api system but it could be down to the format of your bitcoin address you are passing in.

66f4dec6ab29cd056ce1814c6fdfee05e1524b36 is read as a valid bitcoin address using this service

However when I search that address under a block explorer I get the following address which starts with the more common 1 1APPGSyMsPaBUAHtjEm6tPqV9U54JRzmd6

66f is not a start of a bitcoin address i recognise, though there are many out but no mention on the wiki

Thus I would suggest trying your code with the more common bitcoin address format

$FEES = $wallet-&gt;coinSelection(\Blocktrail\SDK\Wallet::normalizeOutputsStruct(['1APPGSyMsPaBUAHtjEm6tPqV9U54JRzmd6' =&gt;'100000']),false,false,\Blocktrail\SDK\Wallet::FEE_STRATEGY_LOW_PRIORITY['fee'];
echo "$FEES";


And see if that works