Hi Ed_T,
For help with merging cells, check the restrictions with the steps below found in the article here: Merge or unmerge cells in Numbers on Mac
"Merge or unmerge cells in Numbers on Mac
Merging table cells combines adjacent cells into a single cell. Unmerging cells that were previously merged retains all the data in the new top-left cell.

Here are some restrictions:
- You can’t merge nonadjacent cells or cells from different areas of the table—for example, cells from the body and the header.
- You can’t merge columns or rows.
- You can’t split a cell. If a cell has never been merged with another cell, it can’t be unmerged.
Note: You can’t use forms for tables with merged cells. Forms in a Numbers for iPhone or iPad spreadsheet are hidden when you open the spreadsheet in Numbers for Mac. In Numbers for Mac, merging cells in a table linked to a form may clear the form."
If you continue to have an issue, reach out to the Numbers support team directly.
Contact Apple Support
Contact Apple for Support and Service
Best.