an open position

This phrase means a job that is available. It's usually used to describe corporate or office jobs.

In the example above, the order is reversed: "there's a position open" instead of "open position". This sentence could also be written:

There is a position that is open now that she left.

However, in normal spoken English, "that is" should be left out.

digital marketing

"Marketing" is an area of business that focuses on how to present a company's products to customers. "Digital" means having to do with computers. So a "digital marketer" is someone who plans how to present the company's products on websites, mobile phones, online videos, etc.

now that (something has happened)

This phrase expresses a situation that started when something happened, and then continues until the present. In the example above, the situation is that there's an open position. The event that happened was that Vicky left the job.

The event should be the cause of the situation. For example:

Now that he's retired, my dad can spend a lot more time working on his garden.

The speaker's dad's retirement is the reason that he can spend more time working on the garden.

If the situation isn't related to the event, use "since (something happened)":

My dad's been having trouble sleeping since he retired.

In this sentence, the speaker's dad's sleep problems aren't related to his retirement; they just started happening in the time period after he retired.