How to downgrade from MacOS Catalina to Mojave
Before you begin

The first thing to understand is that downgrading means you'll need to:


erase your MacBook's drive, and since you don't have a Time Machine backup you'll need to make one (or back up your data in some other way), before downgrading
have an external drive that you can totally erase as part of the process.


Downgrading from Catalina to Mojave

To downgrade, follow these steps:


Use Time Machine (or some other backup method) to back up your data
On your MacBook, use Safari (or another web browser) to open this link:


https://apps.apple.com/ph/app/macos-mojave/id1398502828?mt=12


The Mac App Store should open up on the macOS Mojave page
Download the macOS Mojave installer
Connect your external drive to your MacBook
Use Disk Utility to totally erase the external drive as follows:


leave the drive name as Untitled
format it as either Mac OS Extended (Journaled) or APFS
use the GUID Partition Map

Open Terminal
Enter (or copy and paste) the following command into Terminal


sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia –volume /Volumes/Untitled

NOTE: If at Step 6 you decided to give the external drive a different name, then replace the Untitled bit at the end of the command with whatever you called your drive


Press enter
When prompted enter your admin password (note you won't actually see it appear on screen)
Press enter
When prompted, press the  Y  key to confirm 
Press enter
Terminal will start displaying what it's doing - wait until you see the Done. message
Shutdown your MacBook
Reboot your MacBook while holding down the option key
In a short while you'll see some startup disk options
Now select and boot from the external drive 


(Note: your external drive name will be Install macOS Mojave as the Terminal command we used earlier automatically changed it from Untitled).


Use the Disk Utility option to erase the internal drive (preferably in APFS format and making sure you use the GUID Partition Map option)
Once complete, exit Disk Utility to follow the on-screen instructions to install macOS Mojave onto the internal drive of your MacBook
Now use the Time Machine (or other) backup you made at Step 1 to copy across your data