Why can't I edit this cell?

The person who looks after this spreadsheet has put some guardrails on it so that it keeps working for everyone. You don't need to install anything — these are ordinary Google Sheets™ features.

“You are trying to edit a protected cell or object”

That cell holds a formula or a header that the whole sheet depends on. If you see a warning you can still click OK — but please don't type over a formula: it stops updating. If it's locked, ask the sheet's keeper (the Sheet rules tab says who).

“The data you entered … violates the data validation rules”

That column only accepts certain values — a choice from the list, a date, a number. The message says which. Pick from the dropdown arrow in the cell rather than typing.

A cell turned red

Something in it doesn't fit the column: a typed number where a formula should be, a value not in the list, text in a date column, a duplicate ID, or a required cell left blank. Correct it and the red goes away.

Please don't

• Paste into a dropdown column — pasting skips the dropdown. Choose from the list instead.
• Sort one column on its own — select the whole table, or use a filter view.
• Leave a filter on — use Data › Filter views so only you see the filtered rows.
• Insert rows below the totals — insert them inside the table so formulas and dropdowns come with them.

Keep a spreadsheet of your own that people keep breaking? Shared Sheet Guardrails is free to check.