why am I getting Exec format error when I am writing my linux service?

add shebang to the script

#!/bin/bash
sudo java -jar "/home/ubuntu/FirstWebAppWithoutDB.jar"

and execution permission

chmod +x spring-start.sh

Your spring-start.sh is executed by bash you need to explicit your ExecStart in springboot.service file like this : ExecStart=/bin/bash /home/ubuntu/spring-start.sh