First and foremost, check to see if your Selection Filter has Text enabled. This can be accessed by:
Panels (bottom right) ► Properties ► Selection Filter section ► Make sure Text is enabled (blue)
If still unable to select, this issue is currently a bug due to Multi-line text. This is caused when a Text String is created and then the string is manually resized by adjusting the white corner boxes.
A workaround at the moment is to go to: File ► Print Preview.
Then Close the print preview. This should allow for the text to be selected.