Is the husband's terror from a nightmare, unrelated to the apartment, after performing the Istikhara prayer for purchasing an apartment, an indication that buying the apartment is not obligatory?

One should perform the Istikhara prayer and its narrated supplication. There is no requirement to see a dream afterward. A nightmare does not indicate anything. One can repeat the Istikhara, consult with people of sound judgment and experience, and then proceed with the matter, accepting whatever Allah decrees, whether the matter is completed or becomes difficult.