Do you mean from Webflow project to Webflow project? If so, make sure both are open and use the keyboard shortcuts to copy/paste (cmd+c cmd+v or ctrl-c ctrl-v)
If you mean from another source of text - best to make sure you have the text unformatted - so copy and paste from notepad for example.
Or if you mean with Rich-Text - see this post and reply from @vincent for some clarification: