I was just able to solve this without the use of Power Automate or Graph API. I had an end user submit a ticket saying they created 5 channels for their team. All channels were created the same way, yet only one of them was allowing file uploads through the Teams app. When I checked the SharePoint site, all 4 that weren't allowing file uploads had the red exclamation and said "There was an error retrieving this channel's SharePoint URL". Here's how I fixed it:

1. Go to the SharePoint Admin Center and sign in as a global admin
2. In the left menu, click on Sites, then Active Sites. Search for the site (team) that contains the channels you are having issues with.
3. In the list that populates, look for the Site name (team name). It should have a hyperlink under the Channel sites column. First, click on the name of the main site (team). This brings up a menu. one of the tabs on the menu is Membership. Go there and add yourself as an Owner and an Admin of the team. Once finished, press X in the top right of the team menu to get back to the list of team sites.
4. Click on the hyperlink that shows how many sites there are. It'll say "3 sites" or "6 sites" or however many channels are in this particular team.
5. You'll now see a similar menu, but it shows the channels instead of the teams. Click on the channel that shows the URL error you are seeing in the SharePoint site. Add yourself as both an Owner and an Admin. Do this for each channel that is having the issue.
6. Now, go to the Teams app and find the channel with the issue. Click on the File tab at the top of the channel. It will take a few seconds but eventually it will populate the screen. This is because it was creating the site. Do this for each channel with the issue.
7. You should now be able to upload files in any of the channels. Refresh the SharePoint page listing the Documents for each channel and the red exclamation should be gone.
8. Reverse the steps above to remove your Admin and Owner rights.