Increase the number of fields in the database

Hi, this is a technical limitation for MySql.
Sum of size for all fields cannot be more than 8126/
Try and reduce size for fields that don't require default size : reference, externalReference could be changed to varchar(20) instead of varchar(100).
Try also to reduce size of fields you added.
At last, you can try a replace some varchar fields with mediumtext (these are blobs and their size is not counted in total size for table). Not that you then won't be able to use the field in list.

Another thing to try, it you defined DB in charset UTF8MB4, try and change to UTFB.
This format uses a bit less space (3 bits per char instead of 4 bits per char) 

You could also change to Latin1, but you'll then need to convert the DB