使用openpyxl编辑电子表格。不能写入单元格，" cell是只读错误"

I'm trying to modify an excel worksheet in Python with Openpyxl, but I keep getting an error that says the cells are read only. I know for a fact that they are not because I've been editing these spreadsheets manually for the past two months and have had no issues. Does anyone have an idea of what might be happening? I'm just trying to get my bearings on editing sheets with openpyxl so it is basic code. I'm trying to modify an excel worksheet in Pyth