"how to set it up as an asset?"

"I saw that a lot of people are saying thatI need one Asset account and a sub-account asset account.

can someone explain why."

Property is a Fixed Asset and it helps to give yourself Clarity. Like This:

123 Easy Street <== Parent level account, do not use

Subaccount: Basis Buildings <== Cost here, split off Land

Subaccount: Improvements <== not ongoing Repairs and Maintenance, but Improvements after purchase

Subaccount: Depreciation

Subaccount: Land <== Land never depreciates