When I use the lasso tool and complete this by connecting it back to the starting point, the line stays solid and doesn't blink. Therefore it is not letting me cut the selected item. How can I fix this?
In GIMP 2.10 - there was a slight change as to how the tool behaves. It doesn't appear to be fully documented in the user manual at present, or at least if it is documented, I couldn't find it.
When you make a selection, it will remain a solid line until you double click inside the selection, or alternatively hit Enter / Return. Then the selection will be committed and will show as marching ants (a moving dotted line).
The reason for this behaviour becomes evident if you make a selection which includes some straight line segments - it's so that the selection can remain editable by clicking and dragging the corner handles, until you actually commit the selection