You may encounter an issue when a transaction is selected, it still says "There is no transaction selected" when adding document. See image below.
From past cases, there may be an Ad Blocker enabled on your browser that is preventing the signing session from opening up. Disabling the Ad Blocker for the duration of the signing should allow you or your clients to continue with the signing. You can also try to switch to another browser which does not have an ad blocker enabled. Click the link below for instructions on how to disable the ad blocker for major web browsers.