Many location now require a negative PCR test to let you enter their territory through various travel means, such as public transport including planes, buses and trains, or sometimes even with personal vehicles.
While you should always check the latest requirements before planning any travel and double check a few days before leaving, it might be mandatory to present a negative PCR test, which will prove that, at the time of the test, you are not presenting any sign of contagion from the Coronavirus.
The PCR test is currently being seen as the most viable way of checking for Coronavirus positivity, and is the test required for travel needs.
The FRANKD test isn't valid for travel either, but is cheaper and as reliable as the RT-PCR test.
If you are wondering Where Can I FLY without PCR test, the best is to have a look at the constantly updated and official Reopen Europa website, for flights within Europe.
In most locations, however, you will need a negative PCR test result 48h before flying out.
While there is no work-for-everyone solution, if you are paying for a health or travel insurance, they might actually refund your RT-PCR test in some specific situations, such as getting a necessary test to go back home from abroad, or getting a mandatory RT-PCR test for business purposes.