Help with LOOKUPVALUE

Anonymous - Hard to be certain. The error you are getting is generally encountered when creating a measure and referencing columns without an aggregator. You can try the MAXX(FILTER(...),...) approach:

Days in Stage = MAXX(FILTER(TAB_Projects, TAB_Projects[Project #] = TAB_Summary[Project #] && TAB_Projects[Stage] = TAB_Summary[Stage]),TAB_Projects[Days in Stage])