Learn French: Tu vas tomber dans le gouffre, à moins de fixer ton baudrier. - You are going to fall into the chasm, unless you secure your harness.

Because the person falling and the person securing the harness are the same: tu. When the subject is the same in both parts of the sentence, French uses à moins de + infinitive. You would use à moins que + subjunctive if the subjects were different (for example, if a guide were securing your harness).

Since fixer looks like 'to fix', does it mean repairing the harness?

No, this is a classic false friend! In French, fixer means to fasten, attach, secure, or set in place. If the harness were broken and needed to be repaired, you would use the verb réparer.

Could I use sans (without) instead of à moins de (unless) here?

Yes! Saying ...sans fixer ton baudrier (...without securing your harness) is grammatically correct and conveys the exact same idea. However, à moins de puts a slightly stronger dramatic emphasis on the condition itself.

Since this is a 'negative condition', why isn't there a negative word like pas?

The phrase à moins de inherently creates the negative condition (unless / except if) all by itself. Unlike à moins que, which is often followed by a formal, untranslated ne (the 'ne explétif'), à moins de + infinitive is self-sufficient and does not use any extra negative words.