Different internship programs have different expectations, so it's hard to really say what they'd be looking for.
My best recommendation is to brush up on what you already know, especially as it concerns the language(s) the internship focuses on.
It may be tempting to try cramming more knowledge before the assessment, but I wouldn't actually recommend that; assessments are meant to ensure you have the minimum prerequisite knowledge to thrive in the internship, and if you don't have that, a few days of cramming won't make up the difference. If you turn out not to be ready, take heart and try again in a few months, once you have some more knowledge. Internships aren't like normal jobs in that, if you aren't ready the first time, you're usually welcome to try again later.
Skills aside, the main thing internships look for is someone who is teachable and able to learn. As long as you present that attitude, and can write some code, I suspect you'll have a good chance.
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