@HK89 assuming that in 2021 you were a US person ( citizen/Green Card / Resident for Tax purposes :(
(a) whether you file a 1040 as Married Filing separate or Single would not make much difference taxwise
(b) However, filing as single when you were actually married -- assuming that you were married in a country whose marriage cert is recognized by the USA, is perjury and you could be penalized.
Because of the above reasoning, I would strongly recommend that you file an amended return to correct the error.
Is there more I can do for you ?
