basic filing sharing between Macbook Pros not working

<< I think Mac1 kept switching back to my guest wireless network and I didn't realize it >>

it may be getting a different IP address on a different network

what is typically offered in that box is not the Approved address but the one you have most recently typed, whether they actually worked or not.

There is also a (Browse) button there, which will look around your network and may "Discover" the other Mac.

"Discovery" requires IPv6 set to "link-local ONLY" (not OFF or ON) on every interface you expect to be using.

Greetings coolpontiac,

Welcome to Apple Support Communities. We suggest setting up and using iCloud Drive to share files between the Mac computers. The steps to do this are found in the Apple Support article: Set up iCloud Drive - Apple Support

"On your Mac

1. Go to Apple menu  > System Preferences, then click Apple ID. If you’re using macOS Mojave or earlier, you don’t need to click Apple ID. 
2. Select iCloud.
3. Sign in with your Apple ID, if you need to.
4. Select iCloud Drive.

To add the files from your Mac Desktop and your Documents folder to iCloud Drive, turn on Desktop and Documents. 

• You can then find your Desktop and Documents folder in the iCloud section of the Finder sidebar. 
• If you add a second Mac Desktop, you can find the files in your Desktop and Documents folders in iCloud Drive, in a folder with the same name as your second Mac. The files from your second Mac won't automatically merge with the files on your first Mac."

Let us know if you have any questions.

Best regards!

It is complaining about a transfer protocol whose name is null.

either omit everything before Mikes...

or type smb://Mikes... or possibly afp://Mikes ...

Server Message Block (SMB) was invented by Microsoft, but Apple liked it better than their own 'chatty' Apple File Protocol (AFP) so they enhanced it a little and adopted it was their server protocol.

Hello coolpontiac,

Looking through the steps provided at Set up file sharing on Mac - Apple Support, make sure you are completing every one of them:

1. On your Mac, choose Apple menu  > System Preferences, then click Sharing .

2. Select the File Sharing checkbox.

3. To select a specific folder to share, click the Add button  at the bottom of the Shared Folders list, locate the folder, select it, then click Add.

The Public folder of each user with an account on your Mac is shared automatically. To prevent a folder from being shared, select it in the Shared Folders list and click the Remove button .

4. Control-click the name of the folder, choose Advanced Options, select desired options, then click OK. 

See Set advanced file sharing options.

5. By default, any user set up on your Mac in Users & Groups preferences can connect to your Mac over the network. A user with an administrator account can access your entire Mac.

To give only specific users or groups access to a folder, select the folder in the Shared Folders list, click the Add button  at the bottom of the Users list, then do one of the following:

• Add users or groups from all users of your Mac: Select Users & Groups in the list on the left, select one or more names in the list on the right, then click Select.

• Add users or groups from everyone on your network: Select Network Users or Network Groups in the list on the left, select one or more names in the list on the right, then click Select.

• Add someone from your contacts and create a sharing-only account for them: Select Contacts in the list on the left, select a name in the list on the right, click Select, create a password, then click Create Account.

6. To specify the amount of access for a user, select the user in the Users list, click the pop-up menu next to the user name, then choose one of the following:

• Read & Write: The user can see and copy files to and from the folder.

• Read Only: The user can view the contents of the folder but can’t copy files to it.

• Write Only (Drop Box): The user can copy files to the folder but can’t view its contents.

• No Access: The user can’t see or copy files from the folder.

There is also additional information there that may help.

Have a great day.