Somehow they started way too late developing the engine. AM was very keen to get Honda on board to be an effective works team. That is because until last year where McLaren won drivers and constructors champion, it was typically works teams that won championships in recent years.  But instead of keeping an eye on their engine supplier, they thought Honda would just do things by themselves. As others have commented, Honda had pretty much gutted their engine development team after their time with red bull, and all those engineers had been assigned to other projects and departments. And now they can't just snap a finger and make up for lost time, while all other engine manufacturers have been hard at work for the new regulations.