Hi Olivia - To my knowledge, you can't combine a filter and a collection. The trick is to change the valuefield to valueexpression and utilize IF statements. Some of your filters (like Project Portfolio ID are confusing me because I don't see the ID number.
Here is what I think you want, but just replace 1234 with the portfolio ID. (Also, I did it so that Rating was on the issue level, if it is on the project level add "{project}." before {DE:Rating}
displayname=RAID items
listdelimiter=<br>
listmethod=nested(issues).lists
textmode=true
type=iterate
IF({project}.{portfolioID}="1234",IF({status}="ARL",IF({DE:Rating}="HIGH",{description},""),""),"")
valueformat=HTML
Hope this helps