The full message is, for example: "Your browser blocked the Instagram tab. Allow pop-ups for Studio, or open Instagram yourself: the text is copied."
The Post on button for each post, such as Post on Instagram, opens the network in a new tab, which some browsers block.
What to do
Allow pop-ups for studio.nexframeai.io. Most browsers show a blocked pop-up icon in the address bar; click it and choose to always allow.
Tap Post on [network] again.
Or open the network yourself and paste: your text is already copied, and any images or videos for the post have downloaded.
On a phone, posts with images or videos open your phone's share menu instead, so this message mostly appears on computers or for text-only posts.