"locked" query

Hoping someone can help this novice!

I have created a query that is based upon two separate tables. In one table
"tblReviewTasks" I have a field [strReviewID]. In another table "tblInv" I am
trying to get the related field [strNPI]. Both tables share [strReviewID] and
these are tied with a relationship and a join property. Both are text fields.
My problem is that whenever I add the second table to my query, the query
becomes "locked" and I cannot enter any data.

1) how can you make queries with joined tables (or queries) so that you can
add data? For some queries I have no problem, but with others I get locked up
and I cannot see any difference between them.

2) Can this be better accomplished using a Dlookup? I am using multiple
fields from "tblReviewTasks" but I only need one field from "tblInv". If this
can be done via a Dlookup in the query, how do I write this for the text
field?

Please address both options so that I have a better understanding.
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Thanks!