Given there is an InnoDB table with no PK, and (at least) one unique key:

CREATE TABLE test_tbl (

  col1 int(10) unsigned NOT NULL,

  UNIQUE KEY uniq (col1)

) ENGINE=innodb;

Attempting to add a PK on the same columns:

ALTER TABLE test_tbl ADD PRIMARY KEY(col1);

Gives the following error:

ERROR 1280 (42000): Incorrect index name 'uniq'

and the PK is not created.

This doesn't happen on latest 10.3 or latest 10.5 from dockerhub, only with 10.4