To put it simply, how to deal with it
Delete the file ".maintenance" in the root folder of your WordPress website.
When operating wordpress on a free AWS server etc., there are many cases where memory and disk are insufficient, which often causes moss to update.
In short, it's an out-of-memory error. (Or an error that the disk is missing)
So, the only solution (to prevent recurrence) is to increase the storage part including memory.
If you want to find a cheap server in AWS, t3-small type is recommended.
This type is the best for blog operation.