There is only one element that does not have any naturally occurring isotopes:

* Beryllium (Be)

Beryllium has only one stable isotope, beryllium-9 (⁹Be). All other isotopes of beryllium are radioactive and have very short half-lives.

While technically, all elements can have isotopes (different forms with varying neutron counts), the term "isotope" usually refers to naturally occurring isotopes.