How can I select and copy a field value?

I copy text from formula fields all the time as well. It works best in ‘Table Display’ view. Here you can simply single click the cell and press ctrl-c

It also works in the Detail View, however, as in your image, you do get an error pop-up. You can still copy the text however by doing the same: clicking in the whitespace to select the formula field (it will get a grey box around it as in your image, and the error will pop up) and pressing ctrl-c

This only works to copy the entire text field, and you cannot select just one part of it. I would love if formula columns displayed selectable text though this is currently not the case. There is also a feature request to make a formula that could copy stuff to clipboard with a button: