I can't unlock my iPhone

That appears to be an Activation Lock and your only option is to return the phone for a refund. The device will never be usable by you unless the original owners credentials are entered.

Activation Lock is a theft deterrent and happens most frequently when stolen devices are sold online, but in some cases the original owner may have not properly reset the device to be sold.

In addition to Activation Lock, there are several other problems that can occur when purchasing from an unknown seller.

The All Too Common SAD Reality of Buying … - Apple Community

As you have found, it is possible to use a phone that has the Find My set to another ID, but eventually it will produce the message your are probably seeing (my translation tool does not work with your image). If you see a "Locked To Owner" message, or one referring to iCloud locked:

Take it back to the person from whom you received it. Get them to do the things in this support article or ask for a refund --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support There is no way of discovering the owner of a device unless they left a message to display on the device when it is turned on. For information about how to request access to a deceased family member's Apple account data, and possibly their equipment, read this support article --> How to request access to a deceased family member's Apple account - Apple Support

Unless you have the account information that set the lock, the original receipt from Apple, or can demonstrate legal inheritance from a deceased owner, you cannot remove the lock; recycle the device. Read this article about how to remove Activation Lock --> How to remove Activation Lock - Apple Support

From the very bottom of Apple Trade In - Apple :

"Does Apple offer recycling?

Yes. Apple Trade In lets you recycle any Apple device (including devices from Apple-owned brands) at any Apple Store and on Apple for free. That includes your batteries and old electronic products as well as free, on-demand packaging recycling for our commercial, education, and institutional customers. When we receive your device, it will be thoroughly inspected to determine if components can be recycled or reused. Whether recycled or reused, all activities relating to the processing of your device will be managed in an environmentally responsible way."

What happens when you try to open the phone? When purchased used it should have been reset to factory settings by the original owner and you should be seeing the "Hello" screen to set it up as a new device.