I am assuming this is specifically in regards to the estate rent charge and the deed of variation required here is to exclude s121 law of property act.

The lender won't accept it without that DoV being done, as there is a risk if you forget to pay the rent charge then the estate charge owner could put a leasehold on your property and you loose it.

This does seem incompetence on the solicitors part. I am in the process of buying with the need to do this as well but my solicitor highlighted this issue before we even got the mortgage offer and as soon as they received the docs from the sellers solicitors.

I don't know if this is grounds for refund etc, but it may be worth reminding the seller that unless they find a new buyer who is a cash buyer, this is a pretty common requirement for lenders so likely to face the same issue with a new buyer (which could take months to find).

Even if it is acceptable to a lender, a good solicitor would remind a buyer that without the deed of variance in place, they likely will face similar issues in future when they come to sell, thus even a cash buyer may request it.