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If I'm understanding you correctly, you and your former husband filed jointly in 2018, got divorced sometime in 2019, and now you are having difficulty getting your stimulus check?
If you don't have to file 2019 taxes then you should be able to use the "Non-Filers: Enter Payment Info Here" option on the IRS stimulus check website. However, if it is giving you an error message then it may be because it's "confused" (for lack of a better word) since you filed jointly with your ex-husband in 2018.
Do you know if your ex-husband received $1,200 or $2,400? There have been reported instances of ex-spouses getting the full stimulus check, which is very well what may have happened in your case, especially since you are having problems with the "Non-Filers" link.
If that is what happened then unfortunately there's not much that can be done about that outside of you and your ex-husband coming to an understanding since the courts are closed and, even if they were open, it would almost certainly be more expensive to have it resolved in court than the full amount of the check is worth.