Hi OmenhoteppIV,
I always get that message as well.
In the end, you "will be charged", but from the free credits google is giving you.
You will even receive a receipt in your gmail saying how much costs were produced for the time you used it, but again, nothing will be charged in your account, but taken from your free trial credits.
If you feel more comfortable, start with just want credit card, let it run for 1 day (that is around 13 euros, so no big deal) and you can monitor how your free 300 dollars start going down