Try running the update manually. You can find and run it from the Updates folder in your data path. If you have Sage 50 installed on a single computer and it is in the default location, then it will be C:\Sage\Peachtree\Company\Forms. Browse to that location, find the latest update (probably Sage50_2019.0.0.Upgrade.exe in this case) and double-click the file name to run it. It would be a good idea to temporarily turn off your anti-virus during the installation.

If you need more detailed instructions, check out the knowledgebase articles How to manually install updates and How to find data path and program path.

If that doesn't work, you will probably need to uninstall Sage 50 and the reinstall. If it was the 2019 update that failed, you can go directly to version 2019 when reinstalling.