Teradata Mapping Manager issues - response (1) by ccurry

Regarding your unselect issue, you need to CTRL+Click in the "data area" of the grid and not on the row numbers. This will only unselect the cel(s) on which you CTRL+Click, leaving the rest of that row highlighted, but functionally the row has been unselected. It is not the best GUI implementation of unselecting a row so I will capture this as a future usability enhancement request.
The USI uniqueness error is a known issue with TMM 2.1.3, when saving mappings without map notes. This has been fixed in TMM 2.1.4. The workaround for TMM 2.1.3 is no always use "save with map note" operations and never use the preference to save without map note prompt.
I do not think your metadata has been corrupted, at least it was not for my testing - but no guarantees as it is a code bug. If you do not want to download the latest TMM and start the tutorial over, I suggest you select all mappings in the auto-match results window, CTRL+Click to unselect the rows mentioned in the tutorial, right-click-> save WITH map note, press Create without typing a map note, then go to map set window and make sure no extra rows were saved on your previous save operation. If so, select the rows that you were not supposed to save and press Delete, Yes to delete them.