This is likely due to the memory limitations of your device. Depending on the content you're trying to create, try the following troubleshooting steps:
- If you're using a large size, try the custom resize option in the Resize panel to make the Post smaller. Something like 1080x1080px is a nice baseline.
- If your Post has a lot of images, try removing some images. Each image adds additional memory.
- If your Post uses many photo filters, try using fewer photo filters or different filters. Some photo filters use more memory than others but all photo filters add to the memory.
If you aren't happy with your result after going through the steps listed, consider saving the Post from the web. A computer should have enough memory to save a complex graphic.