Message conversations disappearing when selected

Hello donaldfromdarien,

We'd love to assist you with using Messages on your Mac. To start, let's make sure everything is set up properly on the Mac by completing the steps below and viewing them here: Set up Messages on Mac - Apple Support

" Set up iMessage

1. In the Messages app  on your Mac, enter your Apple ID and password, then click Sign In.

• Make sure to sign in using the same Apple ID on every device. See the Apple Support article Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch.
• If you forgot your Apple ID or password, or if you need to create an Apple ID, go to the Apple ID account website.

1. Choose Messages > Preferences, click iMessage, then select any of the following options:

• Store your messages in iCloud: Select Enable Messages in iCloud. See Access your messages on all your Apple devices.
• You can be reached for messages at: Select the email addresses or phone numbers that people can use to send you messages.
  • See the Apple Support article Add or remove your phone number in Messages or FaceTime.
• Send read receipts: Select this option so that people who send you messages can see when you’ve read them.
  • You can also send read receipts for a specific conversation.
• Start new conversations from: Choose the email address or phone number that you want to use for starting new conversations.
• This option is only available if you have more than one email address or phone number listed under “You can be reached for messages at.” 
  • Note: This is the information that others see when you send a message. For example, if you have your phone number selected, conversations with your friends show your phone number.

You can also set up name and photo sharing and notifications (such as sounds and alerts). "

Hope this helps!