Hi @Jill-Evarts - to accept AmEx, you will want to make sure that your merchant account is set up to do so. Of note, by default AmEx will be accepted when you set up a merchant account in MyVR, so these instructions also serve to demonstrate disallowing a guest to pay with AmEx.
To check this, head to the Merchant Accounts section of MyVR.
Then click on Actions > Manage:
Then click on Supported Cards:
Then make sure that the AmEx setting is set to yes: