pourquoi c'est pas 'j'avais plein de doutes' ?

Question

avoir des doutes -- > avoir plein de doutes                                                                                              C'est pas comment ça?

Cécile Janty-Davies · Accepted Answer

Hi Coca, 
Just as in English, you can use to be or to have to express doubt - 
J'étais plein de doutes = I was full of doubts
or 
J'avais plein de doutes = I had many doubts
Here, we specifically asked you to use 'être' so, étais would have been the correct answer. The important thing was the correct conjugation in the imparfait of the verb.
We have now changed the example to avoid any confusion.

Maarten Kamp · Answer

Not sure what you are asking. The lesson explains the use of ‘avoir’ in imparfait in this phrase, and “plein de qqc” is a familiar expression for ‘lots of/a lot of’, regularly used in place of ‘beaucoup de qqc’.

https://www.wordreference.com/fren/plein