We are stationed in Guam as of June 2021. Our mailing address with the IRS is here in Guam but we are legal residents of Washington State. Because our mailing changed with the IRS to a Guam address we are not eligible for the ACTC and I need to know what to do to make sure our taxes are not messed up.
It's true that a bona fide resident of Guam cannot claim ACTC on their federal tax return. However, active duty servicemembers whose military State of Legal Residence (SLR) is not Guam, and who are residing in Guam solely due to military orders, are not considered bona fide residents of Guam.
See pages 24-25 of this reference:
https://www.irs.gov/pub/int_practice_units/JTOPUOP_97_07.pdf
Also, simply using a Guam mailing address on your tax return does not make you a bona fide resident of Guam.
The address you list on your return is the one where the IRS can mail you a notice and you will get it. It doesn't need to be the same address that you physically live at.
The mailing address you enter on your return, and your state of legal residence are two completely separate unrelated items. In the pull down menu you select your state of residence as WA. Then your mailing address can be anywhere on the planet.
If your federal military income was your "ONLY" source of income for 2021, then when asked if you earned income in any other state, you answer NO.