That is completely normal. The GUI will let you make a folder but not a file. Any program you run as a normal user (even if that user is part of the administrators group) will not let you save there.
However if you open an administrator command prompt and type
you will find it has created a file test.txt at root of C containing the word "whatever".
echo "whatever" > C:\test.txt
Similarly if you open notepad or many other programs as Administrator (right click on the program, run as administrator) it will let you save in root of C.
What are you trying to do exactly? You can run a program as administrator to do it but otherwise I can't give a sensible suggestion unless you say what the objective is. There are also other issues with running things as administrator so don't take that as a default answer.
Definitely don't take ownership of the whole of the C drive whatever you do though. That would cause no-end of problems.