Hello there @poppythecat27! Welcome to the forums 
I'm sorry to hear that you've encountered some issues during your playthrough of Assassin's Creed: Valhalla. I'll try to assist you with both of these.
Regarding your missing "The Legend of Beowulf" quests, we have an ongoing Megathread for this issue. Can you please try these workarounds to see if this causes the quests to trigger again:
- As you play on PS4, please can you open up the Ubisoft Connect overlay via the main game menu to see if this causes the quests to trigger.
- If you play in online mode, please can you switch to offline mode to see if this triggers the quests to appear in-game for you.
If you're still missing the quests after trying these steps, please can you share your report in the linked Megathread? We can then investigate further.
I'm sorry to hear that you're having trouble looking for Sigfred during "The Rot in the Slums" as well. Would you be able to send us a video which shows the following:
- That "The Rot in the Slums" is your current active quest, as well as the location of the quest marker on the map.
- The door that you're unable to interact with, preventing you from progressing further in the quest.
This will help us to take a closer look at the issue you've encountered in this quest.
Thanks! 
devtrackers.gg