That is because the symlink to that timer file has been removed when you disabled the timer before. Enable that first and execute masking again.

Without root
[nemo@Sailfish nemo]$ systemctl --user enable sms-activation-intex.timer
Now you'll see a symlink has been created message. If you check status of timer you'll se it is enabled.
Execute masking now
[nemo@Sailfish nemo]$ systemctl --user mask sms-activation-intex.timer