You included the snippet swatch.liquid with the name of a product option — 'LETTER' — that does not belong to your product.

Use {% include 'swatch' with 'name of your product option here' %}

Example: {% include 'swatch' with 'Color' %}

This is case-sensitive! Do not put in 'color' if your product option name is 'Color'.

You included the snippet swatch.liquid with the name of a product option — 'LETTER' — that does not belong to your product.

Use {% include 'swatch' with 'name of your product option here' %}

Example: {% include 'swatch' with 'Color' %}

This is case-sensitive! Do not put in 'color' if your product option name is 'Color'.