Hi IlonaW,
How exciting that you're being proactive in taking the next step in your beauty career.
Nail trainers are fantastic to get more practice in nail shaping and nail art techniques and is also great for improving your speed.
Always look for ways to improve your techniques such as online tutorials by qualified nail technicians /educators and getting information from this fabulous forum.
The one thing a nail trainer doesn't show you is client contraindications, buffing and filing pressure and longevity of your enhancements on real clients.
Who did you quality with? There is always the option of getting one to one skill building lessons from a qualified educator who can tell you where you need to improve.
As the previous poster mentioned the best way to build a portfolio is advertise to friends or family, or even Gum tree or buy and sell sites that you are offering free nail enhancements that way you can get photos of your work to show to potential employers. You could even charge a small fee to cover your costs.
Have a look on Google at ideas on how to photograph your work with regards to lighting, hand positioning etc. As a bad picture could also let you down.
Lastly always tell the employer what you can do, what you plan on doing and what you have done rather than what you can't do.
Lastly show your enthusiasm, employers can also help you improve your skills if they like your attitude and see potential.
Good luck!