Like the title says, secretrooms-0.9.5+1.16.5.jar doesn't exist anymore. The mod author is gone, the mod URL in the pack manifest doesn't work, the URL that used to be for the Secret Rooms Fabric mod is now used for the Secret Blocks mod.

The closest I could get is secretrooms-0.9.3+1.16.5 off the Wayback Machine, found here: https://media.forgecdn.net/files/3209/165/secretrooms-0.9.3%2B1.16.5.jar
If support is still ongoing for this pack, can you update to change the required file and URL or just remove it from the pack entirely?

The curseforge launcher allows downloading from archived files from curseforge, of which Secret Rooms exists. If you use the curseforge launcher (or a launcher that allows you to enter a curseforge api key and you are using a valid curseforge api key), you will be able to download all mods in the pack without issue

are you using the curseforge launcher?