How can I convert list into the dataframe?

I wanted to use left join function for these two datasets.
Let me call the upper one as "a", and
the lower one is "b".
But it didn't work, and I found that it's because common variable(column) used to join them are incompatible types. Thus, I wanted to join these by "link_id" but that of a is integer and that of b is character.
Both are data frame as you can see in the pictures, and I thought this would be easy to solve, by running "as.integer(b)". But the error 'list' object cannot be coerced to type 'integer' was returned, despite I saw the data type is dataframe by str...
And I also tried "as.integer(b$link_id)" also, but Warning message: NAs introduced by coercion was returned. But there wasn't any NA value in the dataframe. When I ignore this warning messege and run left_join(a, b), it still returns it can't because of incompatible types of key variable...
So what should I do now? Currently I am thinking the best option is finding the way converting list to datafarme.

Hello. If you could provide a reproducible example, that would allow us to better assist you.