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 Windows To make using Sourcetree as simple yet powerful as possible we embed (bundle) dependencies such as Git, Git LFS, and Mercurial. We strive to keep these...
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