Updating omeka s and error with php

I try upgrade omeka s the last version, but when I run migration, I have this error

Fatal error: Composer detected issues in your platform: Your Composer dependencies require a PHP version “>= 7.4.0”. You are running 7.2.24-0ubuntu0.18.04.17. in /var/www/html/vendor/composer/platform_check.php on line 24

I have ubuntu 18.04.4 lts
when I install php 7.4 with

sudo apt-get install software-properties-common
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
sudo apt-get install -y php7.4

I have this error

sudo apt-get install php7.4
Reading package lists… Done
Building dependency tree
Reading state information… Done
E: Unable to locate package php7.4
E: Couldn’t find any package by glob ‘php7.4’
E: Couldn’t find any package by regex ‘php7.4’

Could help me…

I think the problem you’re having installing newer PHP is that the “ondrej/php” repository you’re trying to use doesn’t support your version of Ubuntu anymore. Full support for Ubuntu LTS versions is just a five-year timeframe, which ended for 18.04 earlier this year.

ok. I understand