While Updating TYPO3 from version 6.2 to 7.6, I got the following error message.

Calculated absolute path to tslib directory does not exist.

Something in the main file, folder and link structure is wrong and must be fixed! A typical document root contains a couple of symbolic links:

* A symlink "typo3_src" pointing to the TYPO3 CMS core.
* A symlink "typo3" - the backend entry point - pointing to "typo3_src/typo3"
* A symlink "index.php" - the frontend entry point - points to "typo3_src/index.php"

Sometimes this may be due to php opcode cache. So by running below code, that clears the opcode cache, the issue can be solved.

<?php
opcache_reset();
?>