You must amend, because you should have filed as married all along. When your spouse does not have an SSN, you still file as married, and you include an application for an ITIN (international tax ID number) with your tax return.
At this point, since you never got an ITIN, you file an amended return to change your status to married, and include your spouse's SSN. Be sure to include all your spouse's world-wide income, even if it was already taxed in another country (you can get a credit for those taxes paid to another country). Because your spouse did not have an SSN as of the original April 15, 2019 filing deadline for the 2018 return, there may be certain tax benefits that require a SSN that will be denied when the IRS processes the return (possibly such as college tuition benefits, I don't have a full list). That's just the rules.
