essential to vs essential for

Essential can be followed by for or to, depending on what comes next. As 'establishing' is considered a noun, you can use either.

Before a noun or noun clause, for and to are both correct, as in these examples:

The river is essential for/to the region's economy.
Food is essential for/to life.
Using high quality materials is essential for/to achieving a good result.

Before a verb, use essential to, as in these examples:

It is essential to use the proper technique.
It's essential to arrive on time.

'tolerance is essential to establishing positive relationships."

you could use:
when
for
in
while
at

. . . depends on whether you prefer a modifying phrase as an adverb or adjective.

"tolerance is essential at establishing" is something I don't thing I've ever come across.

Could you provide an example, if it's not too much trouble, where tolerance, or an abstract noun like it, is followed by "at + noun or gerund"?

Thanks.

You never will come across it in correct English.