DECLARE OR REPLACE varfield_names1 STRING;

SET VAR varfield_names1 = 'field1 STRING';
DECLARE OR REPLACE varsqlstring1 STRING;
SET VAR varsqlstring1 = 'CREATE TABLE table1 (PrimaryKey STRING, Table STRING, ' || varfield_names1 || ')';
EXECUTE IMMEDIATE varsqlstring1;

I took some guess on your intent. It helps if you "print" I.e. SELECT varsqlstrin1 before trying to execute it, so you know you glued together the right statement.