Numbers Transpose not available

Maybe it's in a different spot than you are used to. Here is straight from the Number manual for iPhone.

Rearrange rows and columns in Numbers on iPhone - Apple Support

You can transpose the rows and columns in a table to switch them—row 1 becomes column A (the contents of row 1 move into column A and vice versa), row 2 becomes column B, and so on.

Note: If a table provides the source data for a pivot table in your spreadsheet, or if the table is in a spreadsheet that’s shared with others, the table can’t be transposed.

1. Tap the table, then tap  in its top-left corner.
2. Tap  in the bottom-right corner of the screen, then tap Transpose Rows and Columns.

These are the results of transposing:

• Rows and columns resize to the default height and width for the table.
• Header rows become header columns and footer rows become the rightmost columns.
• The table style and custom style changes you made are preserved, including alternating row colors, gridline settings, table and cell outlines, cell and text style, conditional highlights, hidden rows and columns, and so on.
• Chart data references update so charts reference the same data, even after it’s transposed.
• Merged cells transpose normally, unless they’re in the header row. In a few cases, transposing unmerges merged header row cells.
• Sort and filter rules are discarded, and a linked form is unlinked, if transposing results in a table that’s not a valid form.
• Many formulas and functions update automatically when they’re transposed. Warnings indicate any formulas that can’t be updated.

To undo the transposing and revert all the changes, select the table and transpose it again.

Was the cell locked or the document restricted somehow?