If you select your image using CTRL+A to select all, or select part of an image using the Rectangle select too, and hit copy CTRL+C, the pattern should appear in the Patterns Panel.
It will be the first item in the patterns list, named "Clipboard image". You may have to scroll up to find it. If it doesn't show, you may have selected the wrong layer, or if it has multiple layers, you may need to flatten the image first using Image > Flatten Image
Another way is to export your image as a .pat file, into GIMP's patterns folder, then you can hit the Refresh button in the Patterns panel. Use this method if you want to keep the pattern for use in other projects.
To fill something with a pattern, make a selection first, select the pattern, then do Edit > Fill > With Pattern or use the keyboard shortcut Ctrl+;#
If you want to use the Bucket Bill tool to fill something with a pattern, then you need to select the "Pattern Fill" option in the Tool Options.
Documentation is available on the GIMP site
Bucket Fill Tool: https://docs.gimp.org/2.10/en/gimp-tool-bucket-fill.html
If you want to save the pattern without going through saving the .pat file in the patterns folder, it's possible to select the Clipboard Pattern in the Patterns Panel, and hit the "Duplicate This Pattern" button at the bottom left of the panel.
This will automatically save a .pat copy of the image in the patterns folder for you, so that next time you open GIMP, the pattern will still be there for use in future projects.