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
image
the lower one is "b".
image
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.