Enter the following url into the web address as if you were going to a website (replace with your url):
https://mytenant.sharepoint.com/sites/Pictures/_api/web/GetFolderByServerRelativeUrl('/sites/Pictures/Shared%20Documents')/Files
Do you get a web page with xml for each file in your folder? It should be a webpage with just xml on it listing all your files. Notice how you can see all the xlsx files, such as FileOne.xlsx, FileTwo.xlsx, FileThree.xlsx, etc.
Such as:
<?xml version="1.0" encoding="utf-8"?><feed xml:base="https://mytenant.sharepoint.com/sites/Pictures/_api/" xmlns="http://www.w3.org/2005/Atom" xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices" xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata" xmlns:georss="http://www.georss.org/georss" xmlns:gml="http://www.opengis.net/gml"><id>00000b1d-e00b-0b00-b000-bb00d00d000</id><title /><updated>2023-03-21T17:09:53Z</updated><entry><id>https://mytenant.sharepoint.com/sites/Pictures/_api/Web/GetFileByServerRelativePath(decodedurl='/sites/Pictures/Shared Documents/FileOne.xlsx')</id><category term="SP.File" scheme="http://schemas.microsoft.com/ado/2007/08/dataservices/scheme" /><link rel="edit" href="Web/GetFileByServerRelativePath(decodedurl='/sites/Pictures/Shared%20Documents/FileTwo.xlsx')" /><link rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/Author" type="application/atom+xml;type=entry" title="Author" href="Web/GetFileByServerRelativePath(decodedurl='/sites/Pictures/Shared%20Documents/FileThree.xlsx')/Author" /><link rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/CheckedOutByUser" type="application/atom+xml;type=entry" title="CheckedOutByUser" href="Web/GetFileByServerRelativePath(decodedurl='/sites/Pictures/Shared%20Documents/FileFour.xlsx')/CheckedOutByUser" /><link rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/EffectiveInformationRightsManagementSettings" type="application/atom+xml;type=entry" title="EffectiveInformationRightsManagementSettings" href="Web/GetFileByServerRelativePath(decodedurl='/sites/Pictures/Shared%20Documents/FileFive.xlsx')/EffectiveInformationRightsManagementSettings" /><link rel="http://schemas.microsoft.com/ado/2007/08/dataservices/related/InformationRightsManagementSettings" type="application/atom+xml;type=entry" title="InformationRightsManagementSettings"