The error message indicates that another process is currently using the package manager, preventing you from acquiring the lock to perform package management tasks. This can happen if there's an ongoing update or installation, or if a previous process didn't terminate properly. Here's how to handle it:

Step-by-Step Guide

Wait for the Current Process to Complete

Sometimes, the best approach is to simply wait a few minutes for the current process to complete. If the issue persists, you can proceed with the following steps.

Identify the Process Holding the Lock

Identify the process holding the lock and see if it is still active:

sudo lsof /var/lib/dpkg/lock-frontend

This will show you the process that is currently using the lock. Alternatively, you can use the ps command:

ps aux | grep apt

Terminate the Process (if necessary)

If you determine that the process is not performing any critical operations or it is stuck, you can terminate it. Replace PID with the process ID you found in the previous step:

sudo kill -9 PID

Remove the Lock Files

After terminating the process, you can safely remove the lock files:

sudo rm /var/lib/dpkg/lock-frontend

sudo rm /var/cache/apt/archives/lock

Reconfigure dpkg

Sometimes, dpkg can be left in an inconsistent state. Running the following command can fix this:

sudo dpkg --configure -a

Update and Install Packages Again

Now you should be able to update your package list and install packages without issues:

sudo apt-get update

sudo apt-get install php7.4-mysqli