Is there a legal letter that the mortgage company can send to an estate forcing the executor to transfer....?

What a confusingly-written question.

What does the tenant have to do with the executor of the estate except the executor being the one who is now responsible for managing the property and collecting rent from the tenant?

NO ONE can force the tenant to do anything but pay rent per the rental agreement. And the mortgage lender cannot force the executor to transfer the mortgage into the executor's name - again, what does the tenant have to do with that, anyway?

The only thing the mortgage lender can do is foreclose on the property if the mortgage is not paid.  It is the legal duty of the executor to act in the best interest of the estate, as well as pay all estate debt before any estate assets can be paid out to those named in the will.  Usually the property is sold by the executor.  If there is a written lease, the buyer must honor the lease and the tenant stays until the lease expires.  The only way to 'transfer' the mortgage to the tenant is by selling the property to the tenant and the tenant applies for and is approved for a new mortgage in their name. That's the only way. And NOBODY can force the tenant to buy the property.

they requisted money owing from the estate , its ok you get whats left over from the sale , less debts anyway . Or you can sells the property and pay them whats owing .

no, it doesn't work that way

They would foreclose eventually (assuming mortgage not being paid).  Again, you want to SELL the home to them.  Offer an opportunity for them to buy it, if they choose not to, give notice to vacate and sell.  

The mortgage does not get transferred to the executor's name. They have no liability for any debts of the estate. The estate is required to pay the mortgage until the home is sold or transferred to a beneficiary.  Your tenant would have to get their own mortgage and purchase the home.  Very few mortgages are assumable these days, with the exception of transferring it to a beneficiary.

Talk to the probate attorney to find out what your options are.

"I have a tenant in the home and I want them to get the mortgage in their name"

Then they need to make a purchase offer to the estate and get their own mortgage.

"I want the tenant to get their own mortgage"

Exactly.   So what's the problem?   If they want to buy the house they need to get off their butts and do it.

Er, no.

You literally can't transfer the mortgage to someone else - it's a personal debt.

If you don't want the debt or property, you need to SELL the house. 

It's not possible to transfer mortgages, you have already been told that by people who know the law.

Previously asked and answered!  Mortgages aren't assumable!

EDIT:  Rent to buy doesn't involve a bank mortgage.  A rent to buy is just a lease.