Why am I getting that my emacs tags is not a valid tags table?

Try removing the --sort=yes option.

This solved the problem for me with both ctags versions 5.8 (latest) and 5.5.4 (distributed with my version of Linux).

You can also try the ctags binary that is shipped with Emacs, which is likely in the same directory as the Emacs binary. Note: the command line options are a little different.

Try the -e option -e Output tag file for use with Emacs.