Neither is correct, and timetere isn't a real Latin word. A correct translation depends somewhat on whether the command is directed at one person (e.g., you, the bearer of the tattoo) or the world at large (e.g., those who see the tattoo).
For the former case (audience = one person), you could say Noli messorem timere or Ne messorem timueris. Ne messorem timeas is also OK and would tend to make the expression somewhat impersonal ('One must not fear the fear'). In all three, messorem could be moved so that it's the last word instead of the second word.
For the latter case (audience = more than one person), the corresponding translations are Nolite messorem timere, Ne messorem timueritis, and Ne messorem timeatis.
For each audience, the three versions are interchangeable; so you can go for the version that sounds – or, since a tattoo will be involved, looks – the best to you.
If you wish to avoid thinking about intended audience, you can instead use a passive expression that means 'The reaper is not to be/must not be feared': Messor non timendus est. In Latin, this too has the force of a command. For the sake of brevity, you can omit est. You can also replace non with other negators, such as minime ('not at all'), haud, or nullo modo ('by no means'), if you think they will sound (or look) better.
In all these instances, messor is a solid, literal translation of 'reaper.' If you want something more evocative, perhaps, you could try falciferum instead of messorem in the first set of translations, and falcifer instead of messor in the second. This word means 'the scythe-bearing one.'