Here's why and how to correct it:
* "Les" is redundant: "Ustedes" is already plural and includes the "to them" implied by "les."
* Subject-verb agreement: "Ustedes" requires the verb "gusta" to be conjugated as "gusta" (not "les gusta")
The correct phrase is:
"¿Les gusta el sandwich?" which translates to:
"Do you like the sandwich?" (formal, to a group of people)
You can also use the informal "¿Os gusta el sandwich?" for a more casual tone.