iPhone storage full while playing Solitaire

You likely got a popup scam message. The game is sending you this message. I wouldn't use any game which earns its income from letting scam artists try to trick you into buying tools you don't need or issues you most likely don't have. If your storage really was full, it wouldn't pop up while playing a free game.

Delete the game, is what I would advise you to do. And to make yourself feel better, open the Settings App > General > iPhone Storage and you can see if your memory is low, but I'm willing to bet it's not.

It is a scam message designed to get you to purchase unnecessary software or phish for personal information.

Read more about it here -

Recognize and avoid social engineering schemes including phishing messages, phony support calls, and other scams - Apple Support