Cloud has a Quote Locking functionality that locks the edit options for a quote when its proposal is being reviewed by the customer. When a quote is locked, you or other resources assigned to the opportunity cannot add or remove items or change the payment schedule and taxes. The quote is read-only.
As soon as you share or present the proposal for a quote, Cloud automatically locks the quote.
If you want to unlock the quote to make changes requested by the customer, open it and click "Unlock" in the banner at the top. This will enable all editing functionality in the quote.
You will need the "Unlock quotes" permission in the Opportunities permissions group, or be a member of a group that has it, to unlock the locked quote and relevant proposal.
Because the proposal is a live document, any changes you make to the unlocked quote will be reflected in the proposal. If you want to lock the quote again, you will need to reshare the proposal.