This happens when you check out a repository and the casing in the repository path that you typed does not match the actual repository path. When the casing does not match you're able to check out the repo but you get that error when you commit.
However, it can also occur when the name of your SVN account is entered with the wrong capitalization when making the commit (e.g., "Username: Joe" instead of "Username: joe").
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…