1. Certain fields are not available to choose as a Group By when reporting (ie Short Description, Question, etc.) You may also notice that you ARE able to choose the column in question, when choosing an 'Additional Group By'.
2. When in the list view of a table, you don't have the Group By option available when right-clicking on these column headers as well.
Release or Environment
The cause for this is most likely going to be the data type of the field you are trying to group by. While, you may see that the column shows as a String column, with a max length of 255 or less, it could be assigned to a 'mediumtext' data type in the database for a number of different reasons.
The fact that you can choose the field as an Additional Group By, even when you aren't able to choose it as a Group By, is due to a known issue that isn't being fixed at this moment, due to the complexity of fixing it. PRB1252716
1. If you believe the field you are trying to group by should be assigned to a varchar data type, which is a data type for Strings that have a max length of less than 255 characters, then open a case in HI, and direct the assigned engineer to this KB for reference.
OR...you could select the column you want to group by as an 'Additional Group By' in your report.
2. There is no resolution for not being able to group by in list view, without changing the data type of the column from 'mediumtext' to 'varchar' in the database. A case with Technical Support will need to be opened in HI