Apologies. I realized my error after posting, but the site won’t allow me to delete this post.
When this happens, the App Lounge settings page will have this link showing in red as “Having troubles?”
If it first the problem persists be sure to take the advice to try later, like 12 hours later and have the phone fully charged as battery below a certain level is known to hinder or prevent download.
I have no issues with the update to /e/OS 3.5 on my Teraube 2e. But I have a vague memory that I had to restart the phone at some point after the update. However I don’t remember what made me do that.