gnupg: There is no assurance this key belongs to the named user

Solution 1:

I had the same issue after copying my key pair from one machine to another. The solution for me was the set the trust level of the keys:

gpg --edit-key <KEY_ID>
gpg> trust

You will be asked to select the trust level from the following:

1 = I don't know or won't say
2 = I do NOT trust
3 = I trust marginally
4 = I trust fully
5 = I trust ultimately
m = back to the main menu

I selected 5 since I created the key so of course I trust it ultimately :). It will ask you to confirm your decision:

Your decision? 5
Do you really want to set this key to ultimate trust? (y/N) y

After confirming, quit with:

gpg> quit

You should then be able to encrypt using that key.