Could not load list of meta packages: [Errno 12] Cannot allocate memory

I use AWS free tier.
I've encountered the error message saying 'Could not load list of meta packages: [Errno 12] Cannot allocate memory' after apt-get upgrade error with the message 'fatal -> failed to fork'.
Does anyone know why I got these messages and how to solve the problem.
Thank you.

Could not load list of meta packages: [Errno 12] Cannot allocate memory