What does it mean? Possible incompatibility with another mod?

Thanks for reporting this. It means the mod doesn't know how to plant a particular sapling item, probably for a modded tree. Sorry, I'll make the warning give more info in future versions.

Could you post a list of the mods that are installed? Or a log that shows the warning being spammed?

Could you also share your ‘treeplant-common.toml’ config file?

Ah ok, when I get back to my pc I’ll share it. Some trees with mods I have though, are BYG, BOP, mystical world, autumnity, and atmospheric

Don't worry about getting those logs -- looks like the warning message was erroneously being shown when a sapling couldn't be planted because it was too close to another sapling. Oops! I'll update the mod soon to stop those message from appearing and to give more detailed warnings.

Version 0.1.1 is up and includes the fixes. I'm gonna go ahead and close this issue.