You need to do below mentioned things to become a deck officer:

You can join the merchant navy from age 16 onwards as an officer cadet or marine apprentice and train as a deck rating. You will at least four GCSEs (grades A-C), or equivalent qualifications. These should include English, Maths and physics (or combined science).

You must have good eyesight and be physically fit, as you will have to pass a medical before you can start training. You can also start your career after completing an industry-backed foundation degree, HND or degree at university. These courses usually include on board practical training.

So if you are a 12th repeater then there will be no problem provided you should satisfy the age eligibility criteria.

I hope my answer helps. All the very best for your future endeavors!