My worksheet automatically adds a fill color when I enter text.

[K Sommer]

When I type text in a blank cell, the cell automatically is highlightes with
the bright yellow fill. There is no conditional formatting on this workbook.

 

[מיכ×ל (מיקי) ×בידן]

Check your VBA editor.
You might have an Event Macro which does what, usually, a CF does.
Try to Increase the Macro Security to its maximum level in order to
eliminate the Event Macro from running...
You may, also consider to "rem" all the Macro commands by adding a Preceding
apostrophe to each command.
Micky