digitalWrite(mcp2515.sendMessage(&canMsg1), HIGH);
Let's break down this function call :
digitalWrite() takes 2 parameters. The first is the pin number to write to and the second is the state to set the pin to. So, if you wrote digitalWrite(3, HIGH), for instance, pin 3 would be set HIGH. In your code you have not got an explicit pin number, rather you have the value returned from the call to the mcp2515.sendMessage() function, hence my question
Now, if you don't want a pin to go HIGH why are you using digitalWrite() ?
It looks to me as though you should use
mcp2515.sendMessage(&canMsg1)
which is what is used in the examples