When people see this message it is usually because they are working with a very large canvas, and/or are running out of space or RAM on their device.
It may have been triggered when you added an image to a canvas, or opened a canvas from the Sketchbook Gallery.
The first thing to try is to restart your device. Processes running in the background take up RAM, and over time this can leave less available for active apps to use. Restarting your device will stop these process you didn't know were taking up memory.
The next thing to check would be the size of your canvas - sometimes people end up creating very large canvases with many layers, which end up being huge files for a mobile device to handle. If you have lots of layers, see if you have any you don't need and can remove. Also, see how much storage is available on your device, if you are low on storage it may not be possible to create or open an image file.
Finally, we have heard from some users that they see this message when importing an image from their Downloads folder, or sometimes from Google Drive. If this might be the case for you, try moving the image you want to import into your Documents folder.