The add to cart button is not working with products with no variants or with a single variant but does on multiple variants. I recently had a guy from storetasker change the variant issue so that Shopify that customers had to choose a variant. Those multi-variant products work but now the add to cart isn't working on the rest. I have a pixel union superstore. The guy who fixed the variant issue says his work is fine...now we are at a lose...
Ask storetasker developer for backup theme which he created before start working and then check product page with it.
If issue doesn't exist in backup theme then say him before his work it was working fine.
but if same issue exist in backup theme then find another developer to fix it.
You can also ask theme author but I don't think issue exist with fresh copy of theme.