You installed the ubuntu server 64 bit image on your raspberry pi and need to logon.
This is by design. At first logon you need to change the password.
For this image, which can be downloaded from the raspberry pi website, the default username is ubuntu and the default password is ubuntu too. Here a link: Ubuntu Server 20.04.1 LTS Image
You need to type ubuntu twice, then you will need to change the password.
About the Author
Wim Peeters is electronics engineer with an additional master in IT and over 30 years of experience including time spent in support, development, consulting, training and database administration. Wim has worked with SQL Server since version 6.5. He has developed in C/C++, Java and C# on Windows and Linux in different European countries and different European languages. He writes knowledge base articles to solve IT problems and publishes them on the Lubby Knowledge Platform where he is one of the most important contributors and the main developer.