Joomla problem
If the secondary index existed it would be listed on a second row of type Index under the primary one.
I suggest trying to create it using phpMyAdmin.
To do this you need to click the Go button on the Create index row, which brings up a dialogue. Enter the index name shown in the error message in the box at the top and select the column to be indexed in the selection box at the bottom. Then click Save and it should add that index (using an Alter Table command equivalent to the Create Index one that failed). This works (on a test table) for me.
I've tried doing the same operation on my test table using the create index style command and got the #1142 - INDEX command denied to user … error you experienced. So the problem is that we don't have permission to use that style of command.
From a quick look at the PHP code which installs this add-on I think it checks if the index is already present and only creates it if it doesn't. So I suggest trying installation again after manually adding that index. Hopefully installation will then work.
I suggest trying to create it using phpMyAdmin.
To do this you need to click the Go button on the Create index row, which brings up a dialogue. Enter the index name shown in the error message in the box at the top and select the column to be indexed in the selection box at the bottom. Then click Save and it should add that index (using an Alter Table command equivalent to the Create Index one that failed). This works (on a test table) for me.
I've tried doing the same operation on my test table using the create index style command and got the #1142 - INDEX command denied to user … error you experienced. So the problem is that we don't have permission to use that style of command.
From a quick look at the PHP code which installs this add-on I think it checks if the index is already present and only creates it if it doesn't. So I suggest trying installation again after manually adding that index. Hopefully installation will then work.
Glad to hear that worked.
Google shows many instances of the create index error with this Joomla extension so I think it is a common problem, not Plusnet specific. Possibly this is not in the standard list of grants for non-privileged users. That could be for security reasons. As you've demonstrated the alternative command used by phpMyAdmin works, had that been used in this extension code installation would have been successful.
I don't think this problem is widespread - didn't notice its occurrence in other contexts. However I will be raising it with Plusnet.
Google shows many instances of the create index error with this Joomla extension so I think it is a common problem, not Plusnet specific. Possibly this is not in the standard list of grants for non-privileged users. That could be for security reasons. As you've demonstrated the alternative command used by phpMyAdmin works, had that been used in this extension code installation would have been successful.
I don't think this problem is widespread - didn't notice its occurrence in other contexts. However I will be raising it with Plusnet.