The full message is: "Not enough readable text on that page (it may need JavaScript or a login). Paste the text instead."
Some pages load their text only in a browser, or sit behind a login, so Studio cannot read them.
Other link messages:
"Could not read that page. Paste the text instead."
"That link is not a web page or PDF. Paste the text instead."
"That page did not respond. Check the link, or paste the text instead."
"That page returned an error ([code]). If it needs a login, paste the text instead."
"That page is too large to import. Paste the part you need instead."
"That link cannot be imported. Paste the text instead."
"Enter a full link, starting with https://"
What to do
Open the page yourself, copy the text you need, then in Create choose Text, paste it and tap Use this text.