Can you start practicing acting by yourself?
No - that would be at best pointless and at worst, damaging. You can't possibly judge or assess your own acting. You'd only be teaching yourself bad habits which could be very hard to break in the future. The best you could do is read loads of plays and practice learning lots of lines, and go to see as many good plays in theatres as you can afford. You could also join a good community theatre as a helper and observe the rehearsals and performances - see how it all works.
No one can stop you but you can't really practice more than reading aloud plays to find monologues to perform. Acting is not something you can do alone, you need to have other people to observe and act with. If you want to be an actor in the future follow cogito's advice.
Yes, you can act by yourself, but the problem is judging your performance. This is something most people are really bad at. Nevertheless, memorizing passages from plays or movies is very good practice. Actors with varied skills are more in demand; for instance if you can imitate 7 European accents, you could be in more demand for a role set in Europe than actors who don't have that kind of skill. Horseback riding is still a skill in demand in Hollywood, though not so much in NYC. Stunt work is always in demand, as are martial arts skills. Professional acting students spend most of their time sharpening their skills.
You can record yourself on video and find things you can improve. I've done it sometimes as a musician, to improve my stage persona, and it is amazing the difference between what you think you're doing, and then watching you "from the outside". Of course, as the others say, ultimately art is done for somebody, you need the validation and feedback of some audience, but I think there are many things you can learn of observing yourself too (voice, posture, stronger and weaker points...)
You can do some of the very basics. But note that acting and working on stage needs an audience.