I am a long-term computer geek. This should be obvious and easy. Right now I am frustrated and stumped as to what I may try that could be less than obvious.
You cannot add text. When you click on cell D7 what does it show in the Formula bar? If it shows something like '5, then the apostrophe indicates that the cell is marked as text. See FAQ
Or use Ctrl + F8 - View | Value Highlighting. Text cells are then formatted in black, formulas in green, and number cells in blue, no matter how their display is formatted.
Share the file.
P. S. But why am I telling that to a long-term computer geek?
mark the cell range containing the data to convert
press Ctrl+M or right click on the marked range to get the context menu and select Clear Direct Formatting
if your locale does not use the separator(s) that are required for the text to be converted to number then temporarily apply a number format of a locale that uses the desired separator(s), for example English-US to get the . dot decimal separator; you can then later after the conversion apply the desired final number format of your locale. Either that, or replace the decimal separator, skip to the next example further below.
press Ctrl+H or choose Edit -> Find & Replace from the menu
enter Find: .+
enter Replace: $0
under Other options activate
Current selection only
Regular expressions
hit Replace All
This enters the cell content anew, basically as if it was entered from keyboard, and detects numeric/date/time input.