Ran the Create Las Dataset tool with a Folder as the input and it threw the 999999 error.
My solution.
Ran the Create Las Dataset tool with the same 635 files listed individually in the GUI of the tool, success, worked just fine.
Create LAS Dataset (Data Management)—ArcGIS Pro | Documentation
Did you just select the folder when using the tool or did you follow the method outlined by the tool?
In the Geoprocessing pane, a folder can also be specified as an input by selecting the folder in File Explorer and dragging it onto the parameter's input box.
If you followed the above instructions, then you should report it to Tech Support