html errors – how to fix – need your help.

I´ve got a big problem, i´m using yout template for a lot of sites
and normally I would like to do this again,

but now a programmer from a customer wants me to correct all html errors.
He said it is not normal and I have to correct them, he said that is my
commitment. But when I do this and then I refresh your version (template)
all the errors are there again, because they are in your template.

He is using this: (html checker)
https://validator.w3.org/nu/?doc=https%3A%2F%2Fhb-themes.com%2Fthemes%2Fhighend_wp%2F

I am helpless and do not know what I can do now?
Can you help me or do you have an idea, how I can fix this errors, in my case there are 65,
you can see it here, he wants me to fix them all:

This is my site with errors:
https://validator.w3.org/nu/?doc=http%3A%2F%2Fdgsf.werbeagentur-henstedt-ulzburg.de%2F

Do you have an idea, please? Do you have a possibility?
Greetings Melanie