Cant put border around cell

Hello, I am new to Libre and trying to learn the ropes. I have created a spreadsheet and everything seemed fine until I noticed one cell had a border around it that encompassed the cell next to it, in other words row 4 columns A & B have one border around both cells. I would like a sperate border around A& B as there will be seperate data in each cell (Please see attached screen shots) in the first screen shot row 4 column A & B have a border. In screen shot 2 when I click into row 5 A & B above only have the horizontal borders. I have tried removing the border as well as re-adding, but I am unable to choose column 4A independently of 4B and vice versa. What am I doing wrong?

C:\fakepath\Screen Shot 1 2020-05-30 at 3.13.55 PM.png

C:\fakepath\Screen Shot 2 2020-05-30 at 3.15.24 PM.png

(Edit: activated screenshots -AK )

First of all - do not worry, nothing bad happened.

Place the cursor in cell A4 and look at this button on the toolbar.


Do you see? It is pressed now. This means that you are in a merged cell.

Just unpress this button and see the result.