Learn Dutch: U moet voldoende saldo hebben, anders kunnen we niet afschrijven. - You must have a sufficient balance, otherwise we cannot debit.

Anders acts as an adverb meaning "otherwise" that starts the second clause. Because it occupies the first position, it triggers inversion: the conjugated verb (kunnen) must come immediately before the subject (we).

Why doesn't voldoende get an -e ending here?

The word voldoende (sufficient/enough) is a special adjective in Dutch. It is invariable, meaning it never takes an -e ending, regardless of the gender or plurality of the noun it modifies.

The English translation says "we cannot debit", but doesn't afschrijven need an object, like an amount of money?

Not necessarily. In financial and banking contexts, afschrijven is frequently used without a direct object to refer to the general action of debiting or withdrawing money from an account.

Afschrijven is a separable verb. Why hasn't it been split up into schrijven af here?

Because the second clause uses the modal verb kunnen. The modal verb takes the second position in the sentence, which pushes the main verb (afschrijven) to the very end of the clause in its full, unsplit infinitive form.