Python QGIS tool that replaces layout text labels with attributes from an input table

[buran]

Just a guess, but I would expect that QgsProject.instance().layoutManager().layouts()[0].items() would return some kind of iterable of 2-element tuples (e.g. like dict.items()) in this case l would not be an instance of QgsLayoutItemLabel.
As a side note PEP8 recommendation:

Names to Avoid

Never use the characters 'l' (lowercase letter el), 'O' (uppercase letter oh), or 'I' (uppercase letter eye) as single character variable names.

In some fonts, these characters are indistinguishable from the numerals one and zero. When tempted to use 'l', use 'L' instead.