Can I get a Schengen visa of a layover country without a visa for the destination country?

You can apply for a Schengen visa with such an itinerary, but doing so increases the likelihood that it will be refused.

There are situations where a business traveler goes to a country, spends a few hours in a hotel near the airport, and leaves again. People whose work time is very expensive, and who have to negotiate or sign some contract in person. If such a manager applies for a visa for just one day, it might be granted -- or more likely, the manager would already have a multi-entry visa from previous visits.

If a typical tourist does something like that, the visa officials are likely to think that the application makes no sense and deny it.

My advice: think about what you really want to do, on the long run.

You wrote in a previous question that you want to travel to Spain, and that you have a Romanian visa. Now you write that your Romanian visa was denied, and that you'd like to go to Germany instead. (And possibly onwards to Romania?) You have a file with the Romanian authorities. You may already have a file with the Schengen authorities, or you will have one once you apply for a German visa. These files will be kept for a long time, and it will be very difficult for you to come to Europe in the future if the authorities think that you lied to them.

If you want to travel to Europe, see the sights, and then leave again, start building a travel history in your passports with successful visits to countries in your region. Build your life where you live and document that. In a couple of years, apply for the tourist visa.

If you want to immigrate into Europe and stay, look at the kind of immigrants Europe wants. Mostly highly skilled professionals, but also jobs like healthcare workers. Inform yourself about the rules for all countries and pick the one which fits your profile best. That won't be easy, but coming that way is the safe road to citizenship.