An e-Apostille needs PDF files electronically signed by a UK notary or solicitor.
Do not compress or edit the signed PDF: it breaks the signature.
GRO certificates and DBS or ACRO certificates cannot get an e-Apostille.
How to do it with Requisito Listo
Open «FCDO e-Apostille: is my PDF signed?».
Pick the PDF you were given.
The app tells you whether it has a digital signature and whether the portal will accept it.
At the end you'll see a checklist: size, file weight, background and format. If something doesn't comply, it tells you what to fix before the portal rejects it.