User is trying to change the Charge Amount in an estimate service line (Estimates > Services tab), but on clicking Done, the amount does not save. Also there is no Bill Rate displayed.
In Core, you cannot edit the Charge Amount field in Estimates > Services tab because it is a calculated field. It is calculated as Hours x Bill Rate, where the Bill Rate is brought forward from the Activity Items screen. There is no reverse calculation if you could modify the Charge Amount. You can update the Hours or Bill Rate to change the Charge Amount.
If you are trying to update the Charge Amount because the Bill Rate is not visible for you, it would be better if you change the Hours instead. The Bill Rate is hidden in case your security permissions restrict that in the Estimates screen. If you have to view and edit the Bill Rate, ask your Core administrator to customize your security permissions and give you access to it. Check the Core Help for more on customizing security permissions.