Pages document not opening or saving due to lack of "space"?

I just went through the same thing. Check your storage, finder-general-storage. See what is taking up the most space and try and free up space by removing items you no longer use. Mine was full of data, I tried everything and finally got MacCleaner X to do a deep scan and I got to decide what to toss out. I tossed everything it recommended since it did a great job at choosing files I haven't needed in years. They have a free trial but after the 7 days it's $30 something for the whole year. You can utilize it to get rid of the issue and then cancel, but $39 or less for the year is worth it to me. It freed up almost 30GB of useless data that was making my laptop slow and unable to open certain apps. If your issue is too many downloads or too many apps, it's an easy fix. If it's system data I recommend using the Cleaner or a similar product with the same capacity. I tried to do it myself by getting rid of the Caches but it didn't free up enough space. (finder- top menu next to the apple- go menu- then choose- go to folder option-then type library/caches/- and remove the caches) You don't want to just delete random data since all those blank pages with code determine how the programs or apps function and deleting the wrong ones could cause more grief than wanted. Unless you understand what you're going through which personally I do not. Good Luck.

Do not use so-called cleaner products and particularly Mac Cleaner X as they have caused years of problems reported in the Apple Support Communities. These classes of products tend to vacuum valid files that can damage applications and to some extent, files necessary to a normal operating system.

If your Pages document is stored on iCloud Drive and you have exhausted that storage space, then this failure to open a Pages document has been reported previously, and freeing up iCloud Drive storage resolves the issue. The same issue for any storage medium where the Pages document is stored — because opening a Pages document requires temporary storage. That dialog is telling you what you need to do. Remove unneeded files or offload to another external storage space.