Learn Hardcore Spanish (Latin America): La alerta de saldo bajo funciona bien.

In standard Spanish, alerta is usually treated as a feminine noun when it means an alert or a warning, so la alerta is the safest form to learn.

You may hear other uses in some places or contexts, but for general Latin American Spanish, la alerta is the normal choice.

Also, alerta can be used in other ways, such as estar alerta = to be alert, where it is not functioning as a noun.

What does saldo mean here?

Here, saldo means balance or available credit.

In Latin American Spanish, it can refer to things like:

a bank account balance
prepaid phone credit
money left on a card or account
available funds in an app or service

So saldo bajo means low balance or low credit, depending on the context.

Why is there a de in alerta de saldo bajo?

Spanish often uses de to connect nouns where English might simply stack them together.

So:

alerta de saldo bajo literally means something like alert of low balance
in natural English, that becomes low-balance alert

This is a very common Spanish pattern. For example:

tarjeta de crédito = credit card
mensaje de texto = text message
centro de ayuda = help center