This error happens when you try to upload a larger file than your PHP configuration allows.
In order to fix this issue, you need to edit your server php.ini file, usually located in /etc/ and increase the upload limit.
The 3 directives that you have to edit are:
memory_limit upload_max_filesize post_max_size
So look for them in the php.ini file and increase their values like:
memory_limit = 128M upload_max_filesize = 128M post_max_size = 128M
Then save the file and restart apache/nginx in order for changes to take effect.