Google found that the tax you submitted for US-targeted products, in the tax attribute or in account-level tax settings, was lower than the tax charged at checkout on your site. Since July 2025 Google no longer uses merchant-submitted US tax data, so this issue is mostly historical. When it still appears it points at legacy tax values in the feed or a price that has tax baked in.
Older feeds or integrations still send tax or tax_category values; account-level tax settings created before July 2025 disagreed with the real checkout rate; or the price attribute includes sales tax, so Google's comparison with the landing page fails.