Learn Turkish: Bu mekânda insanlar çok candan. - People are very cordial in this place.

The noun can means soul or life. When you add the ablative suffix -dan (meaning from), it creates the lexicalized adjective candan, which means cordial, warm, or sincere. It literally translates to from the soul, but it functions completely as a standard adjective here, not a noun indicating a source.

Why is there a circumflex over the a in mekânda?

The circumflex over the a is used in certain Arabic and Persian loanwords. Here, it tells you to soften the preceding k, making it sound closer to a ky sound. It does not change the vowel harmony rules for the locative suffix, which remains the back-vowel -da.

Could I just say bu yerde instead of bu mekânda?

You could, and it would perfectly mean in this place. However, the word mekân is frequently used in everyday Turkish to specifically mean a venue, hangout spot, or establishment like a café or bar. If the speaker is at a social venue, mekânda sounds much more natural than the generic yerde.

Since the subject insanlar is plural, why doesn't the predicate adjective candan get a plural suffix?

In Turkish, predicate adjectives do not need to agree in number with the subject. While you could technically add the plural suffix to make it candanlar, it is much more natural and common to leave the adjective in its base form when the plural subject is explicitly stated in the sentence.