"will be unable" is correct and usable in written English. You can use it to express the future possibility of being unable to do something. For example: "The large storm approaching will likely cause a power outage, so I will be unable to finish my assignment on time.".
The Knicks will be unable to match any richer offers.
Viewers will be unable to enter the house.
"We believe commodities will be unable to escape the contagion.
Unfortunately, I will be unable to see it either place.
We will be unable to use such material on GuardianWitness.
Businesses will be unable to expand and grow.
Europe will be unable to afford its extensive social programs without such changes, he warned.
Thanks to Ludwig my first paper got accepted! The editor wrote me that my manuscript was well-written
Listya Utami K.
PhD Student in Biology, Bandung Institute of Technology, Indonesia