I am not a specialist in US process but when we create a US invoice, is it normal to not see the Tax amount column? on non-US subsidiaries I can see this field and put the tax amount

No there would usually be sales tax and in turn this implies tax code column as well (I guess you could have header fields also but never really seen anyone use that) If you mean vendor invoice then its correct that there are no such fields, its an enhancement pending. If it really was customer invoice I suspect maybe its not set up unless it worked before? Work through this SA, it may not show simply because per-line taxes are not turned on: https://suiteanswers.custhelp.com/app/answers/detail/a_id/8054

Hello to explain, when I work on subsidiaries in europe, on invoices when I select my tax code there is a field called Tax Amount and I can change the tax value. When I'm working on a US subsidiary, this Tax Amount field doesn't appear on the line as it does for European subsidiaries.

Yes they have different setup tabs - check the above link 🙂

For US subsidiaries, you can't override the Tax Amount. I am not sure why NS designed it that way.