Excel Table Function (Ctrl + T) Troubleshooting

Hello- this is my first post! I like to think I'm well versed in Excel and look forward to contributing to this community.

I am facing some trouble with Excel's table function. After creating the table (i.e. Ctrl T), Excel won't let me add columns. I select a cell within the table, and have tried to insert columns by 1) Alt + I, C (traditional method); and 2) right click within the table and click insert column. Both motions result in a column appearing, then immediately being deleted.

As additional reference, a similar thing happens when I click the "Total Row" under the "Design" section in the ribbon. However if i manually insert a Sum function it works, and furthermore allows me to select whether I want average, sum, etc.

Any help on addressing this issue would be much appreciated!

E