can't flatten sheet metal cylindrical tank jacket

I have made a part using revolved boss/base and rotated it 359 degrees to keep an opening to flatten it. I have been unable to turn it into a sheet metal part in order to flatten it. I need to determine dimensions of the blank needed to manufacture it. I thought it would be fairly simple but since I am new to the sheet metal feature I have spent days trying to figure it out and watch tutorials on youtube but have failed to figure it out. Any guidance and help will be much appreciated. I am open to any ideas or alternatives. Thank you!

As part of the migration from the legacy Forum to the new SOLIDWORKS Forum, there was an overlap in timing where the new Forum was being loaded with historical data from an export migration, while at the same time, the legacy Forum was still operating during the period of March 12th to March 31st. Due to the overlapping timelines and import-export technical issues, posts during that time-frame could not have their media imported. If you created a post in the legacy Forum during that time-frame and wish to update the post in the new Forum with supporting media, please go to your post in the new Forum, enter into Edit mode and re-add any supporting media you want to insert.

The easiest way to find your posted content is: from the new Forum, click on your profile icon in the top right of the GUI, then select "My Profile" and you will be taken to the "Overview" tab where you can see all of your posting history.

Thank you