In this case, it could be that the folder where the program saves the work in progress does not have enough space to save the whole file.
You can try opening WavePad, go to Options > Audio > Audio Working folder and copy the folder path where the files are being saved, open the folder using your file browser and delete any files you may have there that are no longer needed. You can also select a different folder to save this particular recording and confirm you select a folder with enough free space to store the large file you are trying to work with.