wall gapping

[aimanzawavi00]

how to fix this ?

[rxi.gg]

Fix what?

[ToanDN]

Lower the base offset of the upper walls or raise the top offset/ height of the lower walls.

[mhiserZFHXS]

It looks like you may have attached walls to the floor above.

[barthbradley]

For the portions where the floor edge is flush with the exterior wall face, you can use join geometry to remove the visible linework between them - assuming they are the same material.  

Now, if the Walls the are compound-layered and the exterior layers are supposed to cover the floor edge below (e.g. exterior layers extend downwards), then you can modify the Wall assembly to unlock those outer layers and then, under the Wall properties, type in a negative distance value for Base Extension.

https://knowledge.autodesk.com/support/revit/learn-explore/caas/CloudHelp/cloudhelp/2021/ENU/Revit-A...