Did you follow the 9-17 steps of the tutorial? If yes, then did you successfully dump some cards' info into Serial Monitor using Arduino IDE?

There are many potential ways why it could not work and that's why in the "README.md" file I tried to address all of these aspects including:
-choice of the card to read (it has to be a card operating on 13.56Mhz)
-choice of the card to write to (also 13.56Mhz but also with changeable UID value)
-correct wiring and choice of parts (Arduino Pro Micro 5V version instead of 3.3V, the direction of zener diodes)
-testing methods using "DumpInfo" example from the rfid library

The issue doesn't include any information about these so I can only guess what went wrong...

I'd suggest to:
-try to follow 9-17 steps of the tutorial and post here at which point something goes wrong
-post clear images of wiring
-post information about the card you tried to copy and the blank card you used to write the data

Hi guys! Could you please check if your readers can read 7-byte UID mifare cards? Every 522 reader I've tried doesn't see this type of cards. There is some info online that you can change some inductors to increase field power but I have no luck doing that...

Hello, to be honest I know nothing about rfid... It would be much better to check this library for info about it:
https://github.com/miguelbalboa/rfid

There's at least 1 relevant issue there from what I see:
miguelbalboa/rfid#302