Thank you to all of you for your answers.
It looks like I don't have an electrical issue anymore. I was able to start mi motorcycle, with all my lights, horn, and everything working fine. It took a long time to start it though. I think that the reason why I had that electrical issue was because just before I installed a tachometer; and when I took it apart, my bike had lights again. All the attempts to start it were making sure that I had gas, air filter in good condition, spark, starter motor working, and gas. Anyway.
But even if I was able to start it, it was very hard to do it, and actually last Saturday I tried to start it again, even if the day before I rode my bike some miles. And I wasn't able to start it. The thing I was sure it had was a low compression, even if I bought new piston rings just one month ago. So, I though that a worn piston and cylinder would've been the problem, and I ordered a new big bore kit, which would arrive around tomorrow or day after. Do you have any recommendation when I try to install it? and in general, do you guys have any suggestion about my bike's case?
Here's the thing I ordered:
https://www.ebay.com/itm/172197702728