Hi,
If new property in account was created as GA4, I don't think it is possible to "downgrade" this property to a previous version, that is called Universal Analytics. What you can do, is create a new property and choose to use Universal Analytics.
To create a new property using
Universal Analytics (known as "old version", supporting only web measurement), you have to click on
Show advanced options in Property setup:
Then choose to create a
Universal Analytics property: