As a Roblox developer, it is too hard to know what the issue is, when grouping a model. When a developer selects a model with the “Show Details” setting in the Model tab, all attachments inside a model when drag selecting are selected as well, so when they try to group the model they can’t. This makes sense, though the error that Roblox tells you is vague.
The first time it’s encountered during a session, it tells you:
The issue with the above error message is that it doesn’t tell you why the requested items cannot be grouped together.
The second time it is printed, it tells you the same information, but with a “Show Details…” button, but this doesn’t offer any new information.
If Roblox is able to address this issue, those who end up in this situation (which I imagine isn’t uncommon), developers would be able to understand what the issue is. My suggestion is to change the error message to "“Attachments must be parented to a PartInstance. Turn off “Show Details” in the Model tab to not select attachments.” the first part addresses something the developer might not be aware of, and if they want to continue using the drag selection, inform them of the behaviour of “Show Details” and how attachments can only be drag selected when this setting is on.
Here’s a file where this behaviour happens where a developer may be wanting to group a model and is confused by the error:
ReproDuck.rbxl (13.9 KB)