I am trying to replace the album tag with the value in the length tag

I believe I need to convert the length tag but can not find a function to store as string...

See the help on some more properties, e.g. several incarnations of length.
https://help.mp3tag.de/options_export.html

Yet, I would not replace the data in ALBUM (unless it is meaningless data) as the property %_length% is not really a tag but a property written by the encoder. If you have a field LENGTH, then this is a plain text user-defined field which will not change, even if you modify the file length.

I don't remember how I did this in the past, but I put the length in the album field so I could sort by length in the Poweramp app on my phone. That was why I was trying to do this with new speakers I have in mp3 format.

Thanks,

Try Converter>Tag-Tag for the field ALBUM
Format string: %_length% _ %album%

Like that you keep the data of the album name but still have the length as first part in that field. If you find a better player that can also sort by length, you can strip the then superfluous length easily from album