How can I fix this SVN commit which fails as "out of date"?

I had this issue in Eclipse (Windows 10). It was solved by right clicking on the folder in question > Team > Update to HEAD

Updating the working copy should fix it.

From the command line:

X:\WorkingCopy>svn up

or right-click and choose SVN Update... in TortoiseSVN

When you update, you may come across conflicts, which will be marked in your code with

>>>>>
...
-----
...
<<<<<

You'll need to manually resolve these. When you're done, don't forget to mark the file as resolved.

X:\WorkingCopy>svn resolve \path\to\file

or right-click and choose TortoiseSVN -> Mark as Resolved... in TortoiseSVN