I wrote a piece of text. Then I want to transform it with perspective in order to make it look like it was written on a wall.
But when I hit Enter it doesn't change anything. When I try to edit it again it just says: Selected Layer cannot be transformed with active transformation mode.
I wonder if that is anMac problem, because this is on a Mac.

You can't do that sort of transform on text because it would then no longer be 'text'. You need to convert the text object to a transformable object. The easiest way is to have the text as the only item in the vector layer and then convert to a paint layer with layer -> Convert -> to Paint Layer. Then you can put a selection box around the resulting 'picture' (it's not text anymore) and transform it as you wish.

Wouldn't it be doable by adding a transformation mask?

You can do it by adding a transformation mask to the vector layer, then using a perspective transform, which seems to work well and is repeatedly adjustable. However, I have crashed (total vanishment) krita by doing this in the past and it seems to depend on the order of operations and clicking layers, but I can't duplicate this.

Also, if you do this then try to move the text object around, there are limitations on where it can be moved without losing the content at the edges. This is probably a limitation of the transform execution. If the text is more or less where it needs to be, then using a transform mask will do the job, provided you don't hit the magic combination that causes a crash (which is admittedly rare).

EDIT: Strikeout: I was dragging the original text object off the canvas (without realising it) so the content of the transform disappeared at the off-canvas edges.

Oh, that sounds interesting. We should be able to reproduce that

I suspect that those 'total vanishment' bugs depend on many factors which are difficult to duplicate. However, here's one for you:

If you try to Copy a transform layer then you get this error message:
-------------------------------------------------------------
Krita has encountered an internal error:

SAFE ASSERT (krita): "allowAsChild(newNode)" in file /home/appimage/workspace/Krita_Release_Appimage_Build/krita/libs/image/kis_node.cpp, line 480

Please report a bug to developers!

Press Ignore to try to continue.
Press Abort to see developers information (all unsaved data will be lost)
-----------------------------------------------------------
If you press Abort, krita closes with no 'developers information' displayed.

If you press Ignore, you can continue and you can paste the transform layer onto another layer and it will work correctly for a few seconds before it seems to 'blow up'.

Are you supposed to be able to copy and paste transform layers?

The strange thing is that you can drag and drop a transform layer from one paint layer to another, or even from a vector layer to a paint layer and the transform has effect, but that effect last for a few seconds before the 'blow up' effect occurs.

Yes, I can reproduce that. I guess we need a proper bug report for it.

https://bugs.kde.org/397796