Guevara agreed that the law of value remained under socialism but argued that measures taken by the Revolution to undermine the capitalist market meant that the law could not serve as the dynamic catalyst to productivity and efficiency in the same way as it did under capitalism.[8] Socialisation of the means of production and distribution had ‘blunted’ the tools of capitalism.[9] Marx described a commodity as a good which changes ownership, from the producer to the consumer. Consistent with this definition, Guevara insisted that products transferred between state-owned enterprises did not constitute commodities because when they were transferred from one state factory to another there was no change in ownership. The state itself should be considered as one big enterprise.[10] For Guevara commodity-exchange relations between factories threatened transition, via ‘market socialism’, to capitalism. He stressed central planning and state regulation as substitutes to such mechanisms.

Enphasis on:

For Guevara commodity-exchange relations between factories threatened transition, via ‘market socialism’, to capitalism. He stressed central planning and state regulation as substitutes to such mechanisms.

Why develop? We understand that the capitalist categories are retained for a time and that the length of this period cannot be predetermined, but the characteristics of the period of transition are those of a society that is throwing off its old bonds in order to move quickly into the new stage. The tendency should be, in our opinion, to eliminate as fast as possible the old categories, including the market, money, and, therefore, material interest - or, better, to eliminate the conditions for their existence.’

It was partly because material incentives became the main way to motivate people, the relationship between firms was set up in a way that it was about each firm being responsible for its own profits and losses, they had to purchase their inputs etc, and it encouraged firms to do dodgy things so they could be like “oh look we beat our target, bonus pls” etc

His planning system did have some material incentives but his idea was that it would be phased out and people should be motivated using moral incentives. To him the law of value should ‘fade away’

Using his analysis, the USSR didn’t collapse because it had a planned economy. It collapsed because its planning system undermined socialist consciousness, its leadership lost touch with the masses, and it developed a class who had a material interest in undermining the state, due to keeping the law of value.

https://www.marxists.org/subject/economy/authors/yaffeh/che-critic.htm