Maybe you need to OP yourself before you can place blocks? Not sure what could be the cause really since I'm no expert with server stuff but that's the first thing that comes to mind, but it depends on the mods you have too it might be one of those, best to check what might be behind it.
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'Niche Community Content Finder' for fun and support of creators. I look for mods that interest me or need a bit of acknowledgement or bug fix testing/reporting. I also make modpacks/maps
Creator/Maintainer of Fabric, Rift & Tesla Mod Discussion Threads/Niche Mod Youtuber.