Primo Raj's questions

I have asked a question to install sagemath on ubuntu here. But After complete installation it says :

Package configuration


Postfix Configuration                                                             │ 
  │ Please select the mail server configuration type that best meets your     
  │ needs.                                                                    
  │                                                                           
  │  No configuration:                                                        
  │   Should be chosen to leave the current configuration unchanged.          
  │  Internet site:                                                           
  │   Mail is sent and received directly using SMTP.                          
  │  Internet with smarthost:                                                 
  │   Mail is received directly using SMTP or by running a utility such       
  │   as fetchmail. Outgoing mail is sent using a smarthost.                  
  │  Satellite system:                                                        
  │   All mail is sent to another machine, called a 'smarthost', for          
  │ delivery.                                                                 
  │  Local only:             
No configuration:                                                        
  │   Should be chosen to leave the current configuration unchanged.          
  │  Internet site:                                                           
  │   Mail is sent and received directly using SMTP.                          
  │  Internet with smarthost:                                                 
  │   Mail is received directly using SMTP or by running a utility such       
  │   as fetchmail. Outgoing mail is sent using a smarthost.                  
  │  Satellite system:                                                        
  │   All mail is sent to another machine, called a 'smarthost', for          
  │ delivery.                                                                 
  │  Local only:                                                              
  │   The only delivered mail is the mail for local users. There is no        
  │ network.                                                                  
  │                                                                           
  │                                 <Ok>  

I don't know what is these? How can I get rid of this?

Recently, I have install Ubuntu 64amd making duak boot with Windows 7 32bit. Everything is ok ,but I'm unable to install sagemath in it. it says " unable to locate the package sagemath-upstream-binary" even when the code is correct. may I know why?

Here are the complete code I have used:

sudo apt-add-repository -y ppa:aims/sagemath
sudo apt-get update
sudo apt-get install sagemath-upstream-binary  

Below are the terminal codes:

kutubuddin@kutubuddin-h81m-s:~$ sudo apt-add-repository -y ppa:aims/sagemath
[sudo] password for kutubuddin: 
Ign:1 http://ppa.launchpad.net/aims/sagemath/ubuntu bionic InRelease           
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [69.9 kB]    
Err:3 http://ppa.launchpad.net/aims/sagemath/ubuntu bionic Release             
  404  Not Found [IP: 91.189.xx.xx xx]
Hit:4 http://in.archive.ubuntu.com/ubuntu bionic InRelease                     
Hit:5 http://in.archive.ubuntu.com/ubuntu bionic-updates InRelease             
Hit:6 http://in.archive.ubuntu.com/ubuntu bionic-backports InRelease           
Reading package lists... Done                                  
E: The repository 'http://ppa.launchpad.net/aims/sagemath/ubuntu bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
kutubuddin@kutubuddin-h81m-s:~$ sudo apt-get update
Ign:1 http://ppa.launchpad.net/aims/sagemath/ubuntu bionic InRelease
Get:2 http://security.ubuntu.com/ubuntu bionic-security InRelease [69.9 kB]
Hit:3 http://in.archive.ubuntu.com/ubuntu bionic InRelease                     
Err:4 http://ppa.launchpad.net/aims/sagemath/ubuntu bionic Release             
  404  Not Found [IP: 91.189.xx.xx.xx]
Get:5 http://in.archive.ubuntu.com/ubuntu bionic-updates InRelease [65.4 kB]   
Hit:6 http://in.archive.ubuntu.com/ubuntu bionic-backports InRelease           
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/aims/sagemath/ubuntu bionic Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
kutubuddin@kutubuddin-h81m-s:~$ sudo apt-get install sagemath-upstream-binary
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package sagemath-upstream-binary

When I'm trying to install Ubuntu 18.04 on my desktop, it shows the following error:

The 'grub-efi-amd64-signed' package failed to install into /target/. Without the GRUB bootloader, the installed system will not boot.

How to fix this error?