You may have been sent a document that uses a LaTeX package that is not included with the TeX Live installation.  The following steps can be used to add the package so the document can be compiled without errors.

The problem can be identified when the document fails to compile and a dialog similar to the following is displayed.

In this case, the missing package is letltxmacro. This error can happen with recent installations when compiling a Beamer presentation document as there has been an update to a package used with Beamer that requires letlxmacro, but this package is not part of the default installation.

  1.  From the Windows Start Menu, locate the TeX Live 2019 group and click the TeX Live Manager shortcut to start the TeX Live Manager program.
  2. After the TeX Live Manager program starts, load the remote repository.  From the File menu select Load repository.  A dialog will appear showing progress.  Wait until the Close button on the Loading dialog is active and then click Close.
  3. The next step is to find the package you want to install.  First, select Package list All so all available packages can be found.  Next, in the search field enter the name of your package.  The list of packages will be updated as you refine the package name.  Click the radio button in front of the package name to select the package to install.  Finally, click the Install marked button to start the installation process.
  4. Wait until the installation is complete, indicated in the Logs dialog that will appear.  The TeX Live Manager will download the package, install it, and then run update the filename database using the mktexlsr program.  When the process is complete the dialog will appear as follows.  Click the Close button.
  5. The File menu can now be used to select Exit and close the TeX Live Manager program.

This completes the steps and you can now compile with the newly installed package.

Steps to add a package if you are using TeX Live 2018 can be found here: Adding a package to TeX Live 2018 (Windows)