Nobody said they can't go to space.
then why say theres a conspiracy about the shape of the earth
No one says there is a conspiracy about the shape of the earth. The Conspiracy, as it's called, refers to the conspiracy that manned spaceflight HAS occurred and has been happening since the 1950s. Those who support it believe that it hasn't, and that those who propagate it do so using a spherical earth not because they're trying to cover up the truth about earth's shape, but because they're trying to continue fabricating a believable space program.
That said, I'm not a conspiracy theorist and I don't fully support that line of reasoning. I think it's interesting, and surely lies have been told over the years, but on the whole I find it a little far-fetched and I believe there must be a more reasonable answer. Perhaps I give people too much credit, I've done that before.