Breaker box one 120 slot left but need one for a second 220 or 240 breaker.
If your panel can take more half-height (sometimes known as "cheater" or "tandem") circuit breakers, you can replace two existing 120V breakers (on the same phase!) with a tandem pair of half-height breakers; this will then free up a slot in your panel that you can then use for a full-height 240V double pole breaker.

If your panel does not accept tandem breakers, or has all its tandem-capable slots filled already, you will need either have the panel replaced or a few circuits moved into a subpanel placed next to the main panel.

If you can't tell what your panel is capable of, the listing label (usually on the inside front cover) should provide a diagram that will mark tandem-capable slots in some fashion (you can also tell by comparing the number of circuits the panel is listed for to the number of physical breaker slots in the panel).

A further issue, as Ecnerwal helpfully pointed out, is that a heavy-duty heater may simply be too much additional load for the panel itself to handle.  In that case, you're stuck.