The process of upgrading your Mac does not automatically update Pages. If you were using Pages '09 (ink bottle icon) on Sierra, which was a 32-bit application and the alias that is the current Dock icon to that older Pages application will likely show what you mentioned. Click on that Dock icon, hold, and then drag it out over your Desktop and release. It should disappear in a puff of smoke.
Open your /Applications folder, and look for the following Pages icon:
Click once on it and then press option+command+i keys to open a Get Info panel. What version is shown? For Big Sur 11.5.2, it should be Pages v11.1. If so, you can click on that application icon and drag it over your Dock where you want it, and then release it. Right-click on that Pages Dock icon and under the options menu, select Keep in Dock.
If you do not have Pages v11.1, sign into the Mac App Store with your Apple ID, and press command+0 to open the past purchases panel. You may see a blue cloud with a down arrow that will download Pages v11.1.