Can US citizens travel to Andorra?

All US citizens can travel to Andorra without any problems or travel restrictions. These days even they are not obliged to show/carry vaccination cards.
As a US citizen you should consider that there is not any airport in Andorra and The best way for US citizens to travel to Andorra is to have a flight to Barcelona in Spain, or Toulouse in France because actually, they are the closest airport to Andorra.
These days there are lots of tours, especially from Barcelona where you can take a tour for a one day trip to Andorra and visit a selection of attractions there.

Andorra tourist visa for US citizens

actually there is not any Andorra tourist visa for US citizens. These days all US citizens can travel to Andorra with just a valid US passport and that is enough.
I have seen that most of the time, even at the border, they are not asking for a passport. But it is strictly recommended that all US citizens carry their valid passports when they are traveling to this country. (Note: no photo or copy is being accepted)
So as a US citizen don't look for applying for an Andorra Tourist visa if you decide to visit this country, just check your passport expiry date and consider a notes for Schengen visa as follows.

Do US citizens need a visa to visit Andorra?

The answer to this question that “Do US citizens need a visa to visit Andorra?" can be yes and could be no.
The answer is No because Andorra is a visa-free country for all US citizens,
And the answer is yes because in order to travel to Andorra, you have a flight to Barcelona in Spain or Toulouse in France, and that means that you should have a valid Schengen visa for visiting Andorra.
In other words, visiting Andorra needs a Schengen visa, not for Andorra itself and for landing in neighboring countries.
So let me give you a clear answer, yes, all us citizens need a visa and passport to visit Andorra and you can not travel to Andorra without a Schengen visa.
It is very important that your Schengen visa for visiting Andorra should be Multiple entries.

Is Andorra part of Schengen 90 day?

As US citizen when you are applying for a Schengen visa, you can stay in the continuously Schengen area for 90 days, but Andorra is not part of the Schengen 90 days.
actually, when you are in Andorra, you are out of the Schengen area and you are in another country that it has its own visa rule. All US citizens can travel to Andorra and stay there for up to 90 days without any visa from Andorra but you should have a multiple-entry Schengen visa.
But why do you need multiple Schengen visas for traveling to Andorra?
Imagine that for visiting the country of Andorra, you have the plan to have this day trip from Barcelona in one day. First, you are landing in Barcelona and at the airport of Barcelona they are stamping your passport as the first entry to the Schengen area. then you are coming to Andorra, at the border of Andorra they should stamp your passport because you are leaving the Schengen area, then you are spending one day in Andorra and on a way back you are coming back to Spain, again at the border they are checking your visa and your visa here should be multiple, otherwise, you can not come back to Spain.

Can I travel to Andorra without a Schengen visa?

It is not possible to travel to Andorra without a Schengen visa. You should apply for multiple Schengen visas first and then come to Andorra.
Your Schengen visa is being checked in Spain or France and it is not being checked in Andorra.

Can you ask for Andorra Stamp at the border?

Yes, absolutely, but they are not stamping by default, you should ask your guide or your driver to go there and ask them to stamp your passport, especially if your 90 days Schengen is important for you.