Hi Lei! That is a bug and was reported at SRCTREE-4691. It has already been solved and the fix will be released in version 2.5.2. In the meantime, as a workaround, you can fill in all the fields (except the username) and leave the username for the end, then the Save button will remain active and you'll be able to add your account.
Let me know if this helped you!
Supported Platforms macOS Sourcetree has a lot to offer and, like many developer tools, finding and using it all can be a challenge, especially for a new user. Everyone might not love ...
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