Here is how I see it.
Wilbur Ross is worth $2.5B and Donald Trump says his own net worth is more than $10B.
In the height of this government shutdown with 800,000 gov’t workers furloughed or working without pay, the US Commerce Secretary says it should be easy for people to get loans on a short term basis. So, following that advice, Trump and Wilbur should apply to get a bank loan for the $5.7B they need for the wall. The loan would be federally guaranteed as Mexico will eventually pay for it.
If they can’t get a loan, all I can say is "I don’t really quite understand why."