I created a Artificer (Battlesmith) / Wizard (Bladesinger), but I cannot prepare Wizard spells in the Character sheet (just the Artificer spells).

It seems the software it is combining the max number of prepared spells into one list, and using the artificer for that list. Note: it is possible to prepare spells from the Builder, only the Character Sheet seems to be malfunctioning.

All the spells you have listed under Artificer are artificer spells, though of course there is a lot of overlap with wizard. Recall that wizards prepare spells that are in their spellbook, and artificers act like clerics and druids and prepare spells from a set list. 11 is the correct number of spells+cantrips for an artificer with those stats. So I think you just still need to go into your wizard spellbook and prepare spells there.

Your Wizard half of the character can only prepare spells from their spellbook. Do you have any spells in your Spellbook? As a 6th level Wiz, you should have a minimum of 16 Wizard Spells in the book, 8 up to 1st-level, 4 up to 2nd-level, and 4 up to 3rd-level. Your Cantrips you just “know” and do not prepare, so those will not appear in your book.

Thanks for your reply, I did learn the spells by putting them in the spellbook, but was used to preparing them from the same list as well. When I clicked on the list below that, it fortunately show the spells, my mistake!