There fundamentally isn't a secure way to encrypt exported files, because if when you run the project it is able to decrypt them, then another tool can replicate the same decryption. For example if you have an encryption key stored inside the project itself, then any sufficiently motivated hacker (or AI assistant) could also find that key and decrypt the assets anyway. Or you could run the project and then extract the decrypted resources directly from RAM.
If you choose 'Bundle assets' when exporting to Windows/macOS/Linux, then Construct at least puts all the assets in a single large file which cannot be casually browsed in the system file explorer. But the file isn't encrypted and with the right tools the assets could still be extracted. The next best protection is copyright law.