Have not been able to backup on Airpot Express since 2/5

Chances are, you don't have an AirPort Extreme......because an AirPort Extreme does not have a built in hard drive.

Chances are, you have a Time Capsule.......which is in effect an AirPort Extreme that includes a built in hard drive that can be used to back up your Mac using Time Machine.

If you do not know what you have.......

Open AirPort Utility (Finder > Applications > Utilities > AirPort Utility)

Hold down the option key on your Mac while you double click on the AirPort device on the screen

Look for the Model

In the example below, you can see that I have an AirPort Time Capsule. If you have an AirPort Extreme, you will see AirPort Extreme next to Model.

If I wanted to start over how do I erase old backup?

Erase the backups on the AirPort Time Capsule drive by.........

Opening AirPort Utility (Finder > Applications > Utilities > AirPort Utility)

Clicking on the picture of the AirPort Time Capsule

Clicking Edit in the upper right corner of the next small window that appears

Clicking the Disks tab at the top of the next window

Clicking Erase Disk

Selecting the Quick Erase option, if it is not already selected

Clicking Erase

Waiting a few minutes so the operation to complete

The AirPort Express base station does not support Time Machine for backups. Did you mean to say an AirPort Extreme or Time Capsule instead?

If so, what errors are you seeing when attempting to run a backup?

The simple method is to erase the old backup and start over.

Since Sierra Time Machine has become less reliable.. and this is worse on Catalina than ever.

Strongly recommend people use an alternative like Carbon Copy Cloner.

Also backup to USB over network is not a great idea. USB drive plugged into the computer direct will be much more reliable and a lot faster.

I backup to the cloud. Do I still need to backup to Time Machine?

Just to be clear .. you are using Time Machine as the backup software. It is not the target of the backup. If you have a Time Capsule (basically an extreme with a hard disk inside) then many people mistakenly call it Time Machine.

A USB drive plugged into an Extreme can be used in place of the Time Capsule but the speed is very slow.. perhaps why you manage so few backups in a day. By default Time Machine should backup hourly.

If you are backing up to Cloud service then you don't strictly need to run Time Machine locally.

However to restore a whole disk from cloud can take a very long time. It is fine if you have sufficient internet speed and / or patience but restore from local backup is usually much faster and less problematic. If you have never done a restore it is well worth having a go. You don't need to restore to internal disk.. plug in a USB drive and restore to that.

If I wanted to start over how do I erase old backup?

From the discussion so far you are running a USB drive on an Extreme??

The best way to erase is plug the disk directly into the computer and erase the disk in disk utility.

Remember the disk must be formatted HFS+ ie Mac OS extended journaled.

There could be a number of reasons for this. See if the following article can help:

• What to Do When Time Machine Is Stuck on "Preparing Backup" - Lifewire

Open AirPort Utility

Click on the picture of the Time Capsule

If things are working, a smaller window will appear

Click Edit in the right corner of the small window

Example: