I have a problem where I cannot get the green debug thing show when I place a navmesh into the level. It is a VR project, it works when I open a new project, pressing P doesn’t do anything. Tried to track down the problem and realized that Show → Navigation is greyed out for some reason. Please help! 
I had same issue with TopDown Template. I used engine that built from source 5.3 version.
I found missing values in DefaultEngine.ini, so I copied Defaultengine.ini values from Launcher version template project under [/Script/NavigationSystem.RecastNavMesh], and now it works.
[/Script/NavigationSystem.RecastNavMesh]
bDrawPolyEdges=False
bDistinctlyDrawTilesBeingBuilt=True
DrawOffset=10.000000
bFixedTilePoolSize=False
TilePoolSize=1024
TileSizeUU=1000.000000
CellSize=19.000000
CellHeight=10.000000
AgentRadius=34.000000
AgentHeight=144.000000
AgentMaxHeight=160.000000
AgentMaxSlope=44.000000
AgentMaxStepHeight=35.000000
MinRegionArea=0.000000
MergeRegionSize=400.000000
MaxSimplificationError=1.300000
MaxSimultaneousTileGenerationJobsCount=1024
TileNumberHardLimit=1048576
DefaultDrawDistance=5000.000000
DefaultMaxSearchNodes=2048.000000
DefaultMaxHierarchicalSearchNodes=2048.000000
RegionPartitioning=Watershed
LayerPartitioning=Watershed
RegionChunkSplits=2
LayerChunkSplits=2
bSortNavigationAreasByCost=False
bPerformVoxelFiltering=True
bMarkLowHeightAreas=False
bDoFullyAsyncNavDataGathering=False
bUseBetterOffsetsFromCorners=True
bUseVirtualFilters=True
bUseVoxelCache=False
TileSetUpdateInterval=1.000000
HeuristicScale=0.999000
VerticalDeviationFromGroundCompensation=0.000000
bForceRebuildOnLoad=True
something like this…
I don’t know this is correct approach or not, but hope it will be helpful for you.
Interesting! I think my problem was related to the project settings. I exported the settings from another project (which didn’t have this problem) and imported them into the problematic one. Still don’t know what it was though