Learn Italian: Scriva il reclamo e lo lasci alla reception. - Write the complaint and leave it at the reception.

Because this sentence uses the formal imperative (giving a command to someone you address as Lei). With formal commands, object pronouns like lo (it) must always come before the verb. If this were an informal command to a friend, the pronoun would attach to the end of the verb: lascialo.

Why do the verbs end in -a (scriva) and -i (lasci) here?

These are formal command forms. To make a formal command, Italian uses a set of endings that are the reverse of the regular present tense. For -are verbs like lasciare (to leave), the ending becomes -i (lasci). For -ere and -ire verbs like scrivere (to write), the ending becomes -a (scriva).

What does lo refer to in this sentence?

lo means "it" and refers back to il reclamo (the complaint). Because reclamo is a masculine singular noun, we must use the masculine singular direct object pronoun lo.

Why do we use alla before reception? Is reception feminine?