It’s more than likely a stability problem.
check stability with a repair hammer, anything lower than 40% will not allow the attachment of another piece.
To increase stability use walls, pillars or a combination of both.
there are guides on how to do this on you tube.
Stability not an issue. Full walls and first layer of slanted roof are set but cannot finish a few of the second layer. Gaps in the middle of already set slanted roof pieces. Even tested building pillars to go straight up through the gaps. Cannot build anything after setting 2-3 pieces
Still sounds like a stability issue try ceiling tiles to pillar see if you can set roof pieces to that. Seems like once you get so tall roofing is problematic without extra support. Had a few buildings slope roof up so far and finished with ceiling tiles.
if stability is not the issue, try restarting the session. I have had similar issues laying foundations, a quick restart for some reason works most of the time.
stability check with repair hammer and pillars, on xbox one I have a similar problem placing building pieces however walking away from the vicinity and returning can solve the issue. a foundation placement on occasion has solved a roof issue over a pillar