Problem Description :
When I launch STAAD.Pro, this error message comes up. Uninstalling and Reinstalling didn't help.

Reason:

This may happen if there is any issue with the configuration file or there is a problem with default settings specific to the user.

Solution :

Close STAAD.Pro CONNECT Edition if it is open.

Rename the folder C:\Users\<user.name>\AppData\Local\Bentley\Engineering\STAAD.Pro CONNECT Edition to something like C:\Users\<user.name>\AppData\Local\Bentley\Engineering\STAAD.Pro CONNECT Edition_1 where <user.name> is your Windows user name. 

Relaunch STAAD.Pro.  
This folder will get re-generated when STAAD.Pro is launched again. This should take care of the crash issue.

In case you made any customization to the interface, you may follow the instructions in the wiki below to retain those first before deleting the folder mentioned above

https://communities.bentley.com/products/ram-staad/w/structural_analysis_and_design__wiki/41423/how-to-retain-customizations-in-staad-pro-connect-edition