cvs-force-dir-tag is a variable defined in `pcvs-defs.el'.
Its value is

If non-nil, tagging can only be applied to directories.
Tagging should generally be applied a directory at a time, but sometimes it is
useful to be able to tag a single file. The normal way to do that is to use
`cvs-mode-force-command' so as to temporarily override the restrictions,

You can customize this variable.