"We tried to reach him but couldn't", Bosz told Sky Deutschland.
Dortmund said a move to the Camp Nou was "not very likely" after they turned down an offer that did not reflect Dembele's "extraordinary" value or the current state of the transfer market.
Dembele, who has made no secret of his desire to play for Barcelona, failed to show up for practice on Thursday.
"We don't know where he is".
Dembele appears keen to push a deal through, however, and although he was present for the team's official photo on Wednesday, manager Peter Bosz confirmed he skipped a training session on Thursday morning.
And sources in Spain also suggest a fee has been agreed in principle with Liverpool which will see Coutinho leave Anfield in a package worth £108.8m.
"Dembele wasn't at training today".
It is also believed that Dortmund have slapped a €150m price tag on Dembele, but Barca are looking to chop off €30 million from it before agreeing on a deal.
Dortmund have insisted that the club would not consider selling the 20-year-old for less than €150 million, with the Catalans seeing Dembele as a replacement for Neymar.
The France global scored 10 goals in 49 appearances last season after joining Dortmund for £13m from Rennes and helped the club win the DFB-Pokal Cup.
Barcelona have been told they will have to pay £130m to secure the signing of Borussia Dortmund's Ousmane Dembele.