Did you look up the SLCSP:

https://www.healthcare.gov/glossary/second-lowest-cost-silver-plan-slcsp/

This helps identify if their plan is considered affordable, if they are eligible for a premium credit, and separately, the issue of Advanced premium credit. Three different things.

When the second column is 0, you are supposed to look up the values:

https://www.healthcare.gov/tax-tool/#/