You’re trying to define the filePath variable by combining a Posix path string with an HFS string.
Try defining the filePath variable like this…
set filePath to (path to library folder from user domain as text) & ¬
tell application "Numbers"
open alias filePath
OR.. My ...
Here is a simplified example, showing minimal code, to replicate what's shown in your OP, and as you can see in the Replies pane of Script Editor in the image further below there are no errors and both display dialog commands returned the expected value.
Example AppleScript code:
set transferValue to 0
tell application "Numbers" to ¬