My tax summary of 2018 is showing a balance tuition transfer amount but when I want to transfer it to my parents, it shows that I've zero amount available to transfer?

CRA only allows unused amounts of tuition to be transferred to a parent or spouse. This means that you must use the tuition credit first to lower your tax owing to zero and then if you have some leftover, you can transfer it. 

The rule is that you can transfer up to $5000 - the amount you used to lower your tax owing to zero. So if you have $20,000 in tuition and had to use $4000 to lower your tax to zero, you could transfer up to $1000.

Check your Schedule 11 to see exactly how much tuition you needed/used and how much is available to transfer.