Ah, the composer made be in the public domain, but the editor is not (since the edition you uploaded is from 2018). Therefore, the edition you uploaded is copyright protected*. If you don't know Phillip Silver and don't have permission from him to uploaded his edition on IMSLP, then stop, because this is a copyright violation. If you do have permission to submit on his behalf, then read on.
Under the "Add file" menu, "Add scan" is for uploading old scores that have fallen into the public domain, whereas "New edition" is for scores for which you have the authority to release them under one of the creative commons licenses. You should've chosen that second option both times (before, I guess you unknowingly chose New Edition for the score and Add Scan for the parts). You see your files have been deleted for this reason. They're strict about this because it's actually not possible to dedicate a work to the public domain in many jurisdictions (that's what the creative commons licenses are meant to solve), and it's important for legal reasons that these things are done correctly.
To establish that you can submit files on someone's behalf, you associate your account with their composer page. In this case, you should create a composer page for Phillip Silver and associate your account with it, but only if you have authorization from him or his estate to release his edition under a creative commons license. The instructions for doing this are here:
https://imslp.org/wiki/IMSLP:Composer_Portal
* If the edition contains no creative additions on the part of the editor and is an accurate reflection of what the composer wrote, then the edition may be considered a scientific edition and will be public domain in the EU in 25 years. Otherwise, it will be public domain 70 years after the death of Phillip Silver (or 50 years in Canada). In both cases, it won't be public domain in the USA for like 90 years or whatever.