Hi and welcome to the forum
Assuming you want the "grey" to take precedence over "green", try this (I am unfamiliar with mac, so I hope the commands are the same/in the same place)
1. highlight the range you want to apply the conditional formatting to (for instance A2:J20)
2. on the home tab, styles, select CF
3. select new rule, select use formula
4. enter $K12="Yes format fill GREEN
Repeat 3 and 4 with $N12= "complete" format fill GREY
This rule should now be the 1st/top rule, and beside it, should be a box to check "stop if true" - check that box
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