I have a account called EQUIPMENT RENTALS I want to have sub Accounts, eg Digger Rental, Trailer Rental. why do I get the error cant have sub account of itself?

Hello there, James36.

I've tried creating sub-accounts in my company file and I wasn't able to get any errors.

Let's just make sure to use a name different from the parent account. Please refer to my screenshot to guide you.

I've also got a helpful article that you can check: How to create a new parent account or subaccount.

Get back to us if you need more help. We're right here for you. 

QB allows 5 sub-accounts including the parent, so really four.  sub accounts must be the same type as the parent too

You should use items, digger, trailer, etc and post them to same account.  Then if you want to see how much you got/paid for the item you run a report on that item