Errors when items are not linked

The expression cannot find a matching input item

The message says the previous node has no information for this expression. The usual cause is a Code node that built new items without pairedItem, or a step in the middle that changed the count so the rows no longer line up.

If you only need one specific row, stop using .item. Use .first(), .last(), or .all() with an index. Indexes start at 0.

{{ $("Customers").first().json.email }}
{{ $("Customers").all()[0].json.email }}

One output item matches more than one input

Summarize, Aggregate, and Merge fold many items into one, or combine branches. The resulting item has several inputs behind it, and .item cannot choose.

Use first, last, or an index on all, and name the row you wanted from before the fold. If the work really is one output per input, do not fold first and then reach back for a single field. Read the field before the summary.

A different field name will not hide this

A field still listed in the input panel only means that step once output that name. The error says the link between items broke. Replacing {{ $json.email }} with another field still uses .item, so the error stays.

When a single item came in, Duaer can point every later item at it. A sample with one item can succeed in the editor and fail in production when many items arrive. Run again with the same count production uses.