To delete the contents of selected cells, press Backspace. To insert a manual line break in a cell, click in the cell, and then press Command Ctrl+Enter. To select only one sheet in a selection, hold down Shift, and then click the name tab of the sheet. To select multiple sheets in a spreadsheet, hold down Command Ctrl, and then click the name tabs at the lower edge of the workspace. To select multiple cells in different areas of a sheet, hold down Command Ctrl and drag in the different areas. You cannot edit the components of the matrix. To create a matrix in which all the cells contain the same information as what you entered on the Input line, press Shift+ Command Ctrl+Enter. To fill a selected cell range with the formula that you entered on the Input line, press Option Alt+Enter.

Try to assign different keys either for LibreOffice, in Tools - Customize - Keyboard, or in your desktop system. Keys that are assigned to the desktop system are not available to LibreOffice. Some of the shortcut keys may be assigned to your desktop system.

Excel shortcut keys select all data