enhanced-terrain-layer VS gm-notes

I got this error too, and it was due to a newer version of the Enhanced Terrain Layer module. Rolling it back to 10.6 fixed the issue. See bug report here.

Enhanced Terrain Layer: https://github.com/ironmonk88/enhanced-terrain-layer

Enhanced Terrain Layer

For difficult terrain there's also modules... modules for everything! :-) We're using the Drag Ruler module and its addon PF2E Drag Ruler Integration for measuring distances directly. Works great. Now, there's a possibility to use that module with another one for difficult terrain, Terrain Ruler (https://foundryvtt.com/packages/terrain-ruler) and/or Enhanced Terrain Layer (https://github.com/ironmonk88/enhanced-terrain-layer).

GM Notes has done well for me. Lets you add a little pinned text box to any object that only the GM can see (button for it's up at the top of the relevant object window)

check GM Notes, its compatible with 9.x

GM Notes

"update to 0.5 · syl3r86/gm-notes@0e5cba0 · GitHub" https://github.com/syl3r86/gm-notes/commit/0e5cba0d4a33122da7e72b3c16ff5d6457ea6d8d

The GM Notes module adds a special notes section to actors, items and more. It doesn't pop them out automatically though. You still need to go into the actor's sheet.

I use the GM notes module (https://github.com/syl3r86/gm-notes). It adds a new tab to all entities like actors, journal entries, items and so on where the GM can write notes which are visible only to himself