On a flow chart it is quite in order to specify that a variable is a character, string integer, decimal or whatever. In fact it might be imperative.
Often the 'rules' simply mean you don't 'declare' what a variable type is by writing executable code.
eg Your chart might show:
Input the students name -> store as a string variable Name -> ...
You probably wouldn't write
std::string Name -> input Name ...
especially because your flowchart might never be implemented in C++, and/or the person reading it might not understand anything about programming/specific languages.
Flow charts flow at a high level. They don't get bogged down in the weeds.