The error message tells you the problem, in less than obvious terms.
I can speculate about the problem a bit, but this would be subject to your verification about the field in question.
In SharePoint lists, the concept of Primary and Foreign Key constraints is dissimilar to standard relational databases. Rather, in order to enforce Referential Integrity between two tables, you have to use Lookup Fields. This is the same Lookup field most Access developers are loath to implement in Access. Unfortunately, the only way to achieve the equivalent functionality of Primary/Foreign Key constraints is through the Lookup field mechanism.
What can, and often does, go wrong here is that inexperienced Access users implement Lookup fields wrong. That's one of the arguments against using them in standard Access tables, by the way. Not that they don't "work" as advertised, but that it is all too easy to get it wrong and create insurmountable hurdles for users.
The problem is, most likely, that instead of saving the Primary Key from the related table as the Foreign Key in the table shown in your screen shot, your Lookup field and the data entry for it are trying to save the text value from the lookup. And that can't be done. Unfortunately, we've seen that particular misstep many times over the years.
I'm guessing your Power Automate problem is similar, although that's outside my direct experience and knowledge.