While the article is rich with information, it just glossed over the main issue: "One classic technique involves injecting a person with a killed virus. Another uses live viruses that have been grown and deliberately weakened, typically by removing specific genes in their RNA or DNA. Both of these strategies take some time, and scientists worry that if they use them on novel viruses, they may not behave the way researchers predict."
But what is exactly taking time? Killing the virus and/or making a weak version of it? And in what way they can behave unpredictably? Alas, so many questions left unanswered :(
And you also monitor them to see if there are adverse effects. Too many adverse effects and you're vaccine isn't safe enough. Adverse effects are like anaphylactic shock, guillain barre syndrome. And that sometimes candidate vaccines both fail to provide protection but make the infection worse.
There is a balancing act between how bad a disease and the risk you're willing to take with the vaccine. Covid19 is probably the worst. It's really dangerous to certain population groups like the elderly and people with certain health issues. And much less so to young healthy people. To get rid of covid19 we're going to have to vaccinate 90% of the world population which means the vaccine needs to be 100% safe.
Anyway you should not be ashamed just because you don't know something.