In most Linux distributions, user names are case-sensitive and can contain both lowercase and uppercase letters. However, some applications or commands may not support user names with capital letters, or may not handle them correctly.
For example, the su command (which is used to switch to another user's account) is usually case-sensitive and requires the exact case of the user name to be specified. If you try to use su with a user name that has capital letters, but the actual user name has only lowercase letters, the command may fail or may not switch to the correct user's account.
Similarly, some applications or scripts that use user names may not be able to handle user names with capital letters correctly. This can be due to the way the application or script is written, or due to differences in the underlying system libraries or configuration.
To avoid these issues, it is generally recommended to use only lowercase letters in user names, or to make sure that the case of the user name is specified correctly when using commands or applications that require it. If you need to use user names with capital letters, you may need to ensure that the application or command you are using is able to handle them correctly.
If you are experiencing problems with user names with capital letters not working as expected, you may want to check the documentation or online resources for the specific application or command you are using, or seek help from an experienced Linux administrator or online community.