There can be a number of things causing problems with patching. I would start with the steps from this knowledgebase article:

You also want to make sure that you didn't rename files, modify configuration files, or download modifications as all of those can cause problems with the patching process.

If that did not work for you, I would next check the space available on your hard drive. The launcher requires about 80 GB of free space to install and patch TERA.

If that doesn't work, I would next check to see if an anti-virus program is interfering with the install. People have been reporting problems with Windows Defender:

If the patching process for the launcher continues to give you lots of problems, you could consider downloading the game through Steam if it available in your region. Steam uses a binary comparison process to patch the game files instead of the zip manifest system our regular launcher uses.

If none of the above methods work for you, please consider creating a support ticket and talking to our customer service representatives. The wait time right now is a little long since we are still catching up on the tickets from the last patch, but it is the best option if the above suggestions do not fix the problem you are having.