you can only sign with your (individual) key which will start with 02 or 04 for compressed and uncompressed pubkeys respectively.
Ahhh! That was it! Now I can encrypt messages.
I did not know there was a master public key vs individual public key.
Here is how to find an individual public key in the Electrum wallet:
1. Click View -> Show addresses
2. Open the addresses tab
3. Find your receiving address in the list (or pick any other address you want to use) and right-click it -> Select Details
4. Your public key is shown in the middle box
My individual public key started with a 03, which I researched and it is also a 32-bit compressed public key.
Thank you for your help!