Even if /etc/portage/package.accept_keywords directory is exists, tatt wants to write file '/etc/portage/package.keywords/archtest'. I think that this should be fixed...
That is not entirely correct. You can specify what your file to unmask packages should be via: unmaskfile=your_file in ~/.tatt. Unfortunately this feature was undocumented :( The default value is /etc/portage/package.keywords/archtest. I also added a check if the configured file can be created if not existent.